Preheat oven to 375 degrees F. Cook pasta according to package directions.
Meanwhile, in a large skillet melt 1 tablespoon of the butter over medium-high heat. Add mushrooms; cook for 2 to 3 minutes or until mushrooms begin to soften. Add garlic; cook for 1 minute more. Transfer mixture to a large bowl. Place the remaining 1 tablespoon butter in the skillet; melt over medium-low heat. Stir in flour. Gradually add milk, mustard, and salt. Cook and stir for 2 to 3 minutes or until thickened and bubbly. Stir in mushroom mixture. Remove from heat.
Drain pasta; add to mushroom mixture. Stir in beans and 1/2 cup of the cheese. Season to taste with pepper. Transfer to an ungreased 2-quart casserole. Sprinkle with the remaining 1/4 cup cheese. Bake, uncovered, about 20 minutes or until heated through.