Prepare your charcoal grill for two cooking zones, if possible or, if using a gas grill, have one zone on high and one on low or indirect heat, depending on your equipment. You want one area with direct high heat and one side with low or no (indirect) heat.
Slice the onion and peppers into strips. Place a large cast iron skillet on the stove over high heat and add the olive oil to coat the pan. Add the onion and peppers and stir to coat. Add the salt and Italian seasoning and stir again. While the vegetables cook, place the sausages on the indirect heat side of the grill. Stir the vegetables often. You want them to soften and develop a bit of caramelization on the onions.
There is no set time for the sausages, as each grilling setup is unique, but cook to an internal temperature of 160°F (about 10-15 minutes). Once the sausages reach temperature, remove from the grill, and let them rest for a few minutes while you prepare the sub rolls.
In the sub roll, add the whole sausage (or sliced sausages if you prefer) in the buns and top with peppers and onions. Add the basil pesto on top and serve! You can add more Italian seasoning, pesto, parmesan, fresh basil and/or red pepper flakes on top if desired. Enjoy!