Fill a large pot with water and bring to a boil. Add the pasta and cook according to package directions.
While the pasta is cooking, slice the steak into thin strips. Set aside.
Slice the onions and peppers into thin strips as well.
Heat a large skillet over high heat and add 2 tablespoons of butter. Toss in the onions and peppers and 1/2 teaspoon of salt. Cook, stirring often, until browned and peppers are tender-crisp. Add the onions and peppers to a large bowl and set aside.
Add another 2 tablespoons of butter to the hot skillet and toss in the sliced steak. Sprinkle on the seasoned salt. Cook, stirring often, until browned and cooked through. Remove from the heat and add to the bowl with the vegetables.
In a small saucepan, melt the remaining 2 tablespoons of butter with the flour. Whisk constantly and cook over medium heat for 2 minutes. Whisk in the milk and continue cooking until thickened, like a gravy.
Remove from the heat and stir in the Swiss and Monterey Jack cheese. The sauce will be very thick.
Drain the cooked pasta, reserving 1/2 cup of the pasta water. Toss the pasta in the cheese sauce and thin as needed with the pasta water, 1 tablespoon at a time. Mix the steak and veggies in with the pasta, toss, and serve.