Melt butter into olive oil in a large Dutch oven over medium heat. Add sliced onions, salt, and pepper. Cook uncovered, stirring every few minutes, for 30 to 35 minutes until the onions are deeply golden brown, very soft, and reduced to about a quarter of their original volume. Lower heat if they start to scorch.

Add minced garlic, red pepper flakes, Worcestershire sauce, and soy sauce to the caramelized onions. Stir constantly and cook for 30 seconds until fragrant.

Pour in the water and half of the evaporated milk (about 3/4 cup). Whisk the cornstarch into the remaining evaporated milk in a small bowl until fully dissolved, then add to the pot. Increase heat to high. Stir in beef bouillon, parsley, thyme, oregano, paprika, and pepper.

Once the liquid reaches a rolling boil, add the pasta. Reduce to medium-high heat and cook uncovered for 20 to 25 minutes, stirring frequently and pressing pasta into an even layer so it stays submerged. Add water in 1/4 cup increments if the pot looks dry before pasta is done. Cook until just al dente with sauce remaining in the pot.

Remove pot from heat. Stir in shredded Gruyere one small handful at a time, letting each addition melt fully before adding more. Stir in Parmesan the same way. Taste and season with salt and pepper. Add a splash of water or milk for a saucier result. Garnish with fresh parsley and serve immediately.