• Preheat your oven to 350 degrees.
• Prepare a 13 x 9 inch pan with cooking spray and set aside.

Ground Meat Sauce

• In a large skillet heat oil until it shimmers or a drop of water flicked in it dances. Add mushrooms and cook until mushrooms are tender. Drain and set aside.
• Add beef and onions and cook until beef is no longer pink.
• Add mushrooms back to skillet.
• Add tomato paste and garlic. Stir until combined.
• Add tomatoes and cook uncovered, until reduced, about 20 minutes.
• Add peppers. Remove from heat and set aside

Pasta

• In the meantime bring salted water to boil in the large pot.
• Add noodles and cook, stirring frequently, for 3 minutes.
• Reserve 2 cups of the cooking water and allow noodles to drain in a colander in the sink.
• Using a large bowl, combine sour cream, cream cheese, cornstarch, and only ½ cup of the cooking water you set aside. Whisk until blended together. Throw in green onions, cooked noodles, remaining 1 ½ cups cooking water, and 1 teaspoon salt. Stir until incorporated.

Putting it Together

• Next, you need to grab your prepared pan. Spread your noodle mixture, as evenly as possible, across the bottom of your prepared pan.
• Spread beef mixture evenly over top of the noodles.
• Sprinkle shredded cheese on top.
• Bake until casserole is bubbling around the edges and cheese is starting to brown, between 25 and 30 minutes.
• Broil for a moment or two to brown cheese further if desired.
• Let stand for 15 minutes before slicing and serving. Enjoy!