- With the rack in the middle position, preheat the oven to 190 °C (375 °F). Line a baking sheet with parchment paper.
- On the baking sheet, toss the sweet potato, onion, and oil. Lightly season with salt and pepper. Bake for about 30 minutes or until the vegetables are tender.
- Slice the buns in half horizontally. Spread the mustard inside the buns. Spread half the cheese on the bottom of the buns. Garnish with the onions, sweet potato slices, and bacon. Top with the remaining cheese. Close the panini.
- In a panini grill or on a lightly oiled ridged skillet, grill the sandwiches over medium heat, pressing lightly, for about 3 minutes per side, or until the cheese has melted. Cut in half and serve with a green salad.