Step 1: Prepare the Onions

• Peel the onions. Cut them in half pole-to-pole (from root end to stem end). Lay the flat side down and slice thinly along the grain (again, pole-to-pole). This helps the onions hold their shape better during the long cooking process compared to slicing crosswise into rings. Aim for uniform thickness (about 1/8 inch). You will have a lot of onions, but they cook down significantly.

Step 2: Begin Caramelizing (The Slow Cook)

• Place your large skillet or Dutch oven over medium-low heat. Add the olive oil and butter. Allow the butter to melt completely.
• Add the mountain of sliced onions to the pan. Sprinkle with 1 teaspoon of kosher salt. The salt helps draw out moisture, aiding the caramelization process.
• Stir well to coat the onions in the fat. Initially, the pan will seem very full.
• Cook over medium-low to low heat, stirring occasionally (every 10-15 minutes) for the first 30 minutes. The goal here is to soften the onions and slowly evaporate their moisture without browning them too quickly. Adjust the heat if necessary – you want a gentle sizzle, not aggressive frying. The onions will gradually soften and reduce in volume.

Step 3: Deepen the Caramelization (Requires More Attention)

• After about 30 minutes, the onions should be significantly softer and starting to become translucent or lightly golden. Now, you’ll need to stir more frequently (every 5-7 minutes) to prevent sticking and burning as the sugars concentrate.
• Continue cooking for another 20-40 minutes (total time: 45-70 minutes, or even longer). Scrape the bottom of the pan as you stir to lift any browned bits (fond) – this is packed with flavor!
• The onions are ready when they are deeply golden brown (think amber or mahogany), significantly reduced in volume, very soft, and smell incredibly sweet and savory – almost jammy in consistency. If the pan becomes too dry or bits are burning, add a tablespoon of water or broth to deglaze, scraping up the browned bits. Be patient; this stage cannot be rushed!

Step 4: Add Aromatics and Deglaze

• Once the onions are beautifully caramelized, push them to one side of the skillet. If the pan seems dry, add a tiny bit more olive oil to the empty side.
• Add the minced garlic and dried thyme (or fresh thyme leaves) to the empty spot. Sauté for about 1 minute until fragrant. Be careful not to burn the garlic.
• Stir the garlic and thyme into the onions.
• Optional Deglazing: Pour in the white wine or vegetable broth. Increase the heat slightly and scrape the bottom of the pan vigorously to dissolve any remaining browned bits into the liquid. Let the liquid bubble and reduce by about half (this takes 1-2 minutes). This adds another layer of flavor.

Step 5: Cook the Pasta

• While the onions are in their final stages of caramelizing or after deglazing, bring a large pot of water to a rolling boil. Add a generous amount of salt (it should taste like the sea).
• Add the pasta and cook according to package directions until al dente (still has a slight bite). Cooking it perfectly al dente is important as it will finish cooking slightly in the sauce.
• Crucial Step: Before draining the pasta, reserve at least 1.5 cups of the starchy cooking water. This water is liquid gold – the starch helps to emulsify the sauce, making it creamy without necessarily needing a lot of cream.

Step 6: Bring it All Together

• Drain the pasta (reserving the water!) and add it directly to the skillet with the caramelized onions.
• Reduce the heat under the skillet to low.
• Add about 1/2 cup of the reserved pasta water and the grated Parmesan/Pecorino cheese to the skillet. If using, add the optional heavy cream now. Add the black pepper and optional red pepper flakes.
• Using tongs, toss the pasta vigorously and continuously. This action, combined with the heat and the starchy pasta water, helps create an emulsified, creamy sauce that coats every strand of pasta.
• Continue tossing for 1-2 minutes, adding more reserved pasta water, a tablespoon or two at a time, if the sauce seems too dry or isn’t creamy enough. The sauce should cling to the pasta nicely.

Step 7: Final Seasoning and Serving

• Taste the pasta and adjust seasoning if necessary. Add more salt, pepper, or red pepper flakes to your liking. Remember Parmesan is salty, so taste before adding much more salt.
• Serve immediately, piled high in warm bowls.
• Garnish generously with chopped fresh parsley and an extra sprinkle of grated Parmesan cheese.