Slice about one inch off the top of each pepper. Seed the peppers.
Spread 1 ½ cups of the marinara sauce and the 2 teaspoons of olive oil (optional) evenly over the bottom of a casserole that’s large and deep enough to hold all the peppers snugly so they remain upright during baking.
Put the seitan, brown rice and all-purpose seasoning in a large bowl and stir well until combined. Stir in 2⁄3 cup marinara sauce and mix to combine.
Spoon one-sixth of the seitan mixture into each pepper, mounding it, if necessary. Spoon 2 heaping teaspoons of the marinara sauce over each pepper. Carefully position the peppers in the casserole so they will remain upright while baking.
Cover and bake for 50 to 60 minutes, or until the sauce is bubbly and the peppers are tender but not mushy. Let cool for 7 to 10 minutes before serving.
Serve the peppers with tricolor pasta on the side and topped with some of the sauce that has accumulated at the bottom of the casserole. Put the remaining sauce in a gravy boat or small bowl to pass around at the table.