Tomato Tartines with Sunflower Seed and Basil Butter

Name: Tomato Tartines with Sunflower Seed and Basil Butter
Ingredients: 1 lb. (500 g) sourdough bread, cut into 1/2-inch (12-mm) slices; Olive oil as needed; 3 to 5 large heirloom tomatoes, cut into 1/4-inch (6-mm) slices; 1 pint (12 oz./375 g) heirloom cherry tomatoes, halved; Kosher salt; 16 Tbs. (2 sticks) (8 oz./250 g) good-quality salted butter, at room temperature; 3 garlic cloves, grated; 1/2 cup (1/2 oz./15 g) loosely packed fresh basil leaves, chopped, plus more for garnish; Zest of 1/2 lemon; 1/2 cup (2 oz./60 g) toasted sunflower seeds; Flaky sea salt and freshly ground pepper