6 large (about 1 1/2 pounds total) portobello mushrooms, each 4 inches in diameter
1 medium red onion, cut into 4 thick slices
6 small ripe tomatoes or 3 medium
half of an 8-ounce container burrata cheese
3-4 tablespoon chopped fresh basil and fresh chives
Directions
Step 1: Prepare a charcoal grill or heat a gas grill to medium heat. Mix 1/4 cup oil, lemon zest and juice, 2 teaspoons mustard and 1/2 teaspoon salt in a small bowl.
Step 2: Gently twist the mushroom stems off the caps. Lightly brush the caps, stems and onion slices with the oil mixture. Grill stems and onion slices directly over the heat, turning once, until golden and tender, about 10 minutes. Transfer to a cutting board.
Step 3: Add caps to a cooler section of the grill, gill side down. Cook, 2 minutes. Flip; brush with oil mixture. Add tomatoes to the grill. Grill until the smooth side of mushroom caps are golden and tomatoes are warmed and softened a bit, about 3 minutes. Remove from grill; place mushroom caps on a serving platter. Use a small serrated knife to cut the tomatoes in half or slices.
Step 4: Chop grilled stems and onion; put into a small bowl. Season with a teaspoon or two of the remaining oil mixture. Divide the filling among the mushroom caps. Top with a portion of the burrata and two tomato halves or slices. Sprinkle generously with 3 to 4 tablespoons fresh basil and chives. Serve while warm or at room temperature.