Preheat the oven to 400 degrees Fahrenheit. Remove the stems of the mushrooms, and carefully scrape away the gills using a spoon. Toss with the olive oil and season with salt and pepper. Place, rounded side down, on a lightly oiled baking sheet. Place in the oven and roast for 8 to 10 minutes, until the mushrooms are beginning to soften and throw off liquid. Remove from the heat and fill with the pesto. Return to the oven and continue to roast until thoroughly softened, about 10 more minutes.