Preheat oven to 450°F. Bring 8 cups water and 2 tsp. salt to a boil in a medium pot.
Separate ciabatta halves, if necessary. Peel and halve shallot. Slice halves into thin strips. Mince garlic.
Once water is boiling, add pasta and stir occasionally until al dente, 8-10 minutes. Reserve 1/4 cup pasta cooking water. Drain in a colander and set aside.
While pasta cooks, top cut sides of ciabatta with 1 tsp. olive oil and half the garlic (reserve remaining for sauce).
Place a large non-stick pan over medium-high heat. Add 2 tsp. olive oil and shallot to hot pan. Stir occasionally until softened, 2-4 minutes.
Add remaining garlic and stir constantly until aromatic, 30-60 seconds.
Add marinara, reserved pasta cooking water, and spinach. Stir occasionally until spinach wilts, 1-3 minutes. Remove from burner.
Stir pasta, Asiago, half the Parmesan (reserve remaining for topping), and a pinch of salt into pan with sauce until combined.
Transfer pasta to prepared casserole dish. Top with mozzarella, remaining Parmesan, and dollops of cream cheese.
Place casserole dish on one half of foil covered baking sheet. Coat a piece of foil with cooking spray and cover casserole, sprayed-side down.
Bake casserole in hot oven, 15 minutes.
Carefully, remove baking sheet from oven. Remove foil. Place ciabatta on empty half of baking sheet, cut-side up. Bake again until cheese is bubbly and ciabatta is toasted, 5-7 minutes.
