Preheat your grill to medium-high heat.
In a large bowl, toss the portobello caps, sliced onion, and bell pepper with the olive oil.
Season generously with salt and pepper.
Place the vegetables on the grill. A mesh grill basket is perfect for preventing the smaller pieces from falling through.
Grill, turning once, until tender and slightly charred, about 8-10 minutes.
During the last 2 minutes of cooking, lightly brush the tomato slices with oil and grill them for about 30-60 seconds per side, just until they have light grill marks.
To assemble, spread about 2 teaspoons (10 ml / 0.3 fl oz) of pesto onto each roll.
Layer with the grilled portobello mushroom, peppers, onions, and tomato slices.
Serve immediately while hot.