Heat large pot of water to boil pasta.
On medium heat, melt 1 tablespoon butter in large frying pan. Brown mushrooms on one side, about 3 minutes. Remove to a small bowl. Lower heat to medium-low and melt 2 tablespoons butter; add onions and sauté until translucent, about 5 minutes. Add onions to mushrooms in small bowl. Heat remaining 3 tablespoons butter in same pan; add All-Purpose Flour Artisan Blend and seasonings, and whisk to combine. Slowly add milk, whisking to prevent lumps from forming. Add onions and mushrooms back into the sauce and continue stirring occasionally as the sauce thickens. It should be very thick, like pudding. Remove from heat and set aside.
Preheat oven to 350°, with rack in center of oven.
Boil Penne Pasta for 8 minutes; add the frozen peas 1 minute before the pasta is done. When cooked, drain pasta and peas, and rinse well with hot water. Add them to the sauce in the large pan and toss gently to completely coat the noodles and distribute sauce. Add the tuna and toss again gently.
Grease an 8 x 8-inch casserole dish with butter or nonstick cooking spray. Scoop pasta mixture into casserole and top with crushed potato chips. Bake at 350° for 30 to 40 minutes, until the casserole is bubbling and the pasta is starting to brown around the edges. Let rest a couple of minutes before serving.
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