Bring a large pot of salted water to a boil and cook the pasta until al dente. Drain and set aside.
Melt 2 tablespoons butter in a large skillet over medium heat. Add the mushrooms and cook until they release their liquid and start to brown.
Stir in half the minced garlic and cook for 30 seconds. Sprinkle the flour over the mushrooms and stir for 1 minute.
Slowly pour in the milk while whisking to avoid lumps. Bring the sauce to a gentle simmer until it thickens slightly.
Add the frozen peas, Parmesan, salt, and pepper. Stir until the peas are warmed through and the cheese melts.
Toss the cooked pasta into the sauce until evenly coated.
While the pasta finishes, mix the remaining tablespoon of butter with the rest of the garlic. Spread the mixture over the bread and place it under the broiler for 2 to 3 minutes until golden.