Melt butter in olive oil in a large 7-quart Dutch oven over medium heat. Add the sliced onions, 1/8 teaspoon salt, and 1/8 teaspoon pepper. Stir to coat in the fat.

Cook the onions, stirring occasionally every 5-7 minutes, until caramelized and dark golden brown, about 30-35 minutes. If onions start to scorch, reduce heat slightly and add additional butter or olive oil.

Once onions are caramelized, add the minced garlic, red pepper flakes, Worcestershire sauce, and soy sauce. Cook, stirring constantly, for 30 seconds until fragrant.

Add the water and half of the evaporated milk to the pot. In a small bowl, whisk the cornstarch with the remaining evaporated milk until smooth, then add to the pot. Increase heat to high.

Stir in the beef bouillon, parsley, thyme, oregano, paprika, and pepper. Stir until bouillon dissolves and mixture comes to a rolling boil.

Add the uncooked pasta and reduce heat to medium-high to maintain a strong simmer. Cook uncovered for 20-25 minutes, stirring frequently and pushing pasta into an even layer so it stays submerged. The pasta should reach al dente with excess liquid remaining. Add water 1/2 cup at a time if liquid evaporates before pasta is done.

Remove pot from heat. Add Gruyere cheese one handful at a time, stirring until each addition melts completely before adding more. Follow with Parmesan, stirring until smooth.

Taste and season with additional salt and pepper if desired. For a saucier consistency, stir in additional water or milk a few tablespoons at a time. Garnish with fresh parsley if desired and serve immediately.