Preheat a barbecue or indoor grill to medium-high. Brush the bread with 2 Tbsp. of the olive oil. Grill the bread until heated through and lightly charred, about 30 seconds per side. Set the bread on a serving platter.
Heat the remaining 1 Tbsp. of the oil in a skillet set over medium heat. Add the mushrooms and garlic and cook until mushrooms are tender, about 5 minutes. Add the lemon juice, pepper sauce, and salt and pepper, cook 1 minute more, and then remove from the heat.
Top each bread slice with an equal amount of the mushrooms. Dot the top of each crostini with 1 tsp. of the pesto and they are ready to serve.