Preheat the oven to 425°F and line a small rimmed baking sheet with parchment paper.

In a medium bowl, whisk together olive oil, balsamic vinegar, onion powder, garlic powder, dried basil, smoked paprika, salt, and pepper until combined. Add the portobello mushrooms and turn to coat in the mixture.

Place the mushrooms on the prepared pan, gill side-up, and spoon the remaining sauce over the gills. Bake for 20 minutes until tender and cooked through.

While the mushrooms bake, make the aioli by blending the mayonnaise, roasted red pepper, garlic, Dijon mustard, and salt until smooth and creamy.

Towards the end of the mushroom baking time, toast the halved ciabatta rolls until golden brown, then brush one side of each roll with the red pepper aioli.