Radish Tartines with Green Butter

Name: Radish Tartines with Green Butter
Ingredients: 8 medium-size mixed radishes (such as Easter egg, black, watermelon, and Purple Ninja) with tops (about 12 oz.); 2 medium-size Japanese turnips with tops (about 2 1/2 oz.); ½ cup unsalted butter (4 oz.), preferably grass-fed, at room temperature; 1 teaspoon finely grated lemon zest, plus lemon wedges for serving; ½ teaspoon flaky sea salt, plus more to taste; ¼ teaspoon black pepper, plus more to taste; 1 small garlic clove, finely grated (about 1/4 tsp.); 6 (3/4-inch-thick) soft rustic bread slices (about 1 1/4 oz. each), lightly toasted; Crushed red pepper, for garnish