Place a cast iron skillet directly over the heat, add a little oil and saute the diced peppers, onion, and garlic for about 15 min. Move the skillet to a cooler area of the grill and add the entire jar of marinara sauce to the skillet. Put the bratwurst sausages directly over the flame. Close the grill dome and cook the sausages for about 3 minutes a side until they are cooked. After they are cooked add the sausages to the skillet and stir everything around. Keep the bratwurst in the skillet along with the peppers and onions for another 15 to 20 minutes.