Preheat grill to 350 degrees F or medium heat.
Slice peppers and onions thin. Add to a bowl with 1 tbsp oil, 1/4 tsp salt and 1/4 tsp pepper. Toss well.
Add the sausages to the grill and cook according to the package instructions.
Add the peppers and onions to the grill on either a grill mat or a vegetable basket. Toss every 2 minutes until charred and softened (about 6-8 minutes).
While grilling, in a bowl, whisk together tomato paste, mustard and mushroom stock until well combined. Add in Italian seasoning, remaining 1/4 tsp salt and 1/4 tsp pepper, stir together.
In a skillet over medium heat, add 1 tbsp oil and saute garlic for 1 minute. Add in the grilled peppers and onions and the sauce. Stir together well and cook until heated through and no liquid remains.
Add the sausages to the bun and top with the onions and peppers mixture.