When I think of portobellos, I think of them as a vegetarian version of steak. I roast them to give them an intense flavor and crisp texture, and add a light balsamic vinaigrette for brightness in flavor.
Preheat the oven to 450 degrees.
Lay the portobello mushrooms on a work surface, bottom side up, and season with salt and pepper, to taste.
In a small bowl, whisk together the balsamic vinegar and olive oil and season with salt and pepper, to taste. Drizzle 1/2 of the balsamic vinaigrette over the portobello mushrooms.
Add the sliced shallot to the bowl with the leftover vinaigrette and toss lightly, then sprinkle the shallots over the mushrooms. Roast in the oven until the mushrooms are a dark, roasted color and have released all of their liquid, for 25-30 minutes.