Position a rack in the center of the oven and heat the oven to 475° F. Line a heavy-duty rimmed baking sheet with parchment. In a medium bowl, toss the mushrooms with enough of the olive oil to coat generously, the salt, and a few grinds of pepper.
Turn the mushrooms out onto the baking sheet and arrange them stem side down. If the mushrooms cover the baking sheet sparsely, arrange them toward the edges of the baking sheet for the best browning. Roast the mushrooms until brown on the bottom, 20 to 25 minutes. Flip and continue to roast until browned on the top, 5 to 10 minutes.
Return the mushrooms to the bowl in which you tossed them with the oil, or put them in a clean serving bowl. If they seem a bit dry, drizzle them with a little oil. Season to taste with salt, pepper, and lemon juice, if using.