Preheat oven to 350°F. Butter 13x9x2 inch glass backing dish. Cook pasta in a large pot of boiling salted water until tender but still firm to bite, stirring occasionally. Drain.

Melt 2 tablespoons butter in large pot over medium-high heat. Add mushrooms and onion; saute until vegetables are tender, about 5 mins. Stir in flour, mustard, salt and pepper; cook 1 min. Gradually whisk in milk. Cook until sauce is smooth and slightly thickened, whisking constantly, about 3 mins. Remove from heat. Add cheese and parsley; stir until cheese melts. Mix in pasta.

Saute bacon in medium skillet over medium-high heat until crisp. Using slotted spoon, transfer bacon to paper towels to drain. Mix half of bacon into pasta and transfer to prepared dish. Sprinkle with remaining bacon.

Melt remaining 2 tablespoons butter in small saucepan over medium heat. Add breadcrumbs; saute until beginning to brown, about 3 min. Sprinkle breadcrumbs over pasta. Bake until pasta is heated through and golden brown, about 20 mins.