Heat oven to 425°.
Heat a large nonstick skillet over medium-high heat. Add olive oil, onion and green pepper. Cook for 5 minutes, stirring occasionally. Set aside.
Spray a 16 x 12-inch baking sheet with nonstick cooking spray. Unroll pizza crust and stretch out to 1 inch from edge of baking sheet. Bake at 425° for 12 minutes. Remove from oven.
Heat broiler. Season steaks with salt and pepper. Broil for 7 to 8 minutes, turning after 4 minutes, until internal temperature of steaks registers 135° on an instant-read thermometer for medium-rare. Let rest for 5 minutes. Thinly slice 1 steak; reserve second steak for another use (such as steak salad).
Spread mustard over crust. Scatter 11/2 cups of both cheeses, the onions, peppers and steak slices over the top. Sprinkle remaining cheese over pizza.
Broil for 1 to 2 minutes, until heated through and cheese is melted. Cut into 8 pieces and serve immediately.