Chop the seitan into quarters and give it a couple pulses in the food processor until it's a similar texture to that of hamburger. Fry in pan with the olive oil over low/medium heat for 4-5 minutes until browned. Set aside.
Cook the pasta shells per package directions for al dente. Drain and set aside.
Melt the butter in a large sauce pan over medium heat and whisk in the flour to create a roux. Add the milk and cheese, stirring constantly until everything is melted and smooth.
Stir in the beefy crumbles, tomato sauce and seasonings. Taste and add S&P as needed. Fold in the pasta, reduce heat to low and let simmer for 1-2 minutes until everything is heated through. Add more milk if it’s too thick. Serve immediately.