Using your hands, crush the tomatoes in a large bowl, breaking up any large bits. Add the garlic, 2 tablespoons olive oil, and salt and pepper.

Heat a charcoal or gas grill to high (alternatively, heat 2 tablespoons oil in a large skillet over medium-high heat).

Toss the shrimp with the remaining 2 tablespoons oil and season with salt and pepper.

Place the shrimp on the grill ( or in a skillet), turning only once, until lightly charred and just cooked through, 3 to minutes total, depending on size. Immediately transfer to the bowl with the crushed tomatoes and butter and toss to coat. Add crushed red pepper flakes, if using. Add the lime juice and let sit a minute or two; the heat from the shrimp will melt the butter, gently cooking the tomato and garlic.

Transfer the shrimp to a large serving platter or bowl, making sure to pour the tomato-y juices over everything. Top with the parsley, serving more limes alongside with warmed tortillas, a bowl of rice, or slabs of toast for mopping up juices.