Melt the butter over low heat in a medium (10-inch) saute pan. Add the olive oil, garlic, rosemary, red pepper flakes, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper and cook over low heat for one minute. Off the heat, zest one of the lemons directly into the butter mixture.
Meanwhile, arrange the shrimp snugly in one layer in a large (12-inch) round ovenproof sauté pan (or 10 × 13-inch baking dish) with only the tails overlapping. Pour the butter mixture over the shrimp. Sprinkle with the sea salt and 1/2 teaspoon black pepper. Slice the ends off the zested lemon, cut five (1/4-inch-thick) slices, and tuck them among the shrimp. Roast for 12 to 15 minutes, just until the shrimp are firm, pink, and cooked through. Cut the remaining lemon in half and squeeze the juice onto the shrimp. Serve hot with the toasted bread for dipping into the garlic butter.