The warmth and ease of a casserole on a cold night meets the subtle and classic flavors of French onion soup in this baked pasta dish. First, you'll brown and caramelize the onions and mushrooms, then mix them together with pasta, cream, and thyme, layering flavor upon flavor. Finally, you'll top the casserole with croutons and cheese (just like the soup!) and bake. Richly cheesy and perfectly indulgent, this dish might even top the classic it was inspired by.
Cut mushrooms into ¼" slices.
Halve and peel onion. Slice halves into thin strips.
Lightly crush croutons into coarse crumbs.
Cook the Mushroom-Onion Mixture
Place a large non-stick pan over medium-high heat. Add 2 tsp. olive oil, mushrooms, onion, garlic, ½ tsp. salt, and ¼ tsp. pepper to hot pan.
Stir occasionally until onion and mushrooms are caramelized and deep brown, 10-12 minutes.
While mushroom-onion mixture cooks, cook pasta.
Cook the Pasta
Once water is boiling, add pasta and cook until al dente, 9-11 minutes.
Drain pasta in a colander, shaking to remove as much water as possible. Set aside.
Reserve pot; no need to wipe clean.
Make the Pasta Mixture
Return pot used to cook pasta to medium-high heat.
Add cream, vegetable base, half the thyme (reserve remaining for garnish), half the mozzarella (reserve remaining for topping), cooked pasta, and mushroom-onion mixture to hot pot.
Stir occasionally until slightly thickened, 2-4 minutes.
Bake the Casserole
Pour pasta mixture into prepared casserole dish. For best results use an 8" casserole dish. Top with croutons and remaining mozzarella.
Bake in hot oven until cheese is golden brown, 10-14 minutes.
Serve family-style, garnishing with remaining thyme. Bon appétit!