Chicken with Lemon and Spicy Spring Onions

Name: Chicken with Lemon and Spicy Spring Onions
Ingredients: 1 lemon; 2 (3 1/2–4-lb.) chickens; Kosher salt, freshly ground pepper; 2 large red onions, quartered; 2 bunches spring onions or 3 bunches scallions, trimmed, divided; 4 green garlic bulbs, very finely chopped, or 4 garlic cloves, finely grated; 1/2 cup extra-virgin olive oil; 1/2 cup (1 stick) unsalted butter; 6–8 (1-thick) slices crusty bread; 1/4 cup parsley leaves with tender stems; Crushed red pepper flakes (for serving)