Heat grill to high. Toss tomatoes in oil and season with salt and pepper. Place on the grill in a grilling basket and grill until charred and softened, turning a few times, about 10 minutes. Remove to a bowl. Crush the garlic with a little salt to form a paste and add to the bowl with a few tablespoons of olive oil.
Brush bread with olive oil. Season with salt and pepper, and place on the grill. Grill the bread until lightly golden brown on both sides, about 20 seconds per side.
Combine the ricotta with a little olive oil in a small bowl and season with salt and pepper. Mound some of the tomato mixture on the bread. Lay prosciutto on top. Dollop with some ricotta mixture and garnish with lemon juice squeezed over the top.