In a large bowl, toss the mushrooms with the garlic, rosemary, lemon zest and the 1/2 cup of olive oil and let sit for 1 hour.
Lightly oil a cast iron skillet, and saute mushrooms over medium-high heat, stirring occasionally until browned (about 8 mins).
Brush bread with oil and broil in oven, turning once, until toasted (about 1 min).
Top the toasts with the mushrooms. Top each with a slice of burrata and serve.