If bread isn't stale, cut into 1-inch cubes and spread on a baking sheet. Let air dry for a few hours or toast in 300°F (150°C) oven for 10-15 minutes until dry but not browned. Transfer cubes to a large mixing bowl.

Heat olive oil in skillet over medium heat. Add chopped onion and cook 5-7 minutes until softened and translucent. Stir in minced garlic and cook 1 more minute until fragrant.

Add grated zucchini to skillet and cook 5-8 minutes, stirring occasionally, until softened and most moisture evaporated. Remove from heat and cool slightly.

Add sautéed vegetables to bread cubes in bowl. Toss in 1 cup grated Parmesan, basil, parsley, salt, pepper, and optional red pepper flakes. Mix gently to combine.

In a separate bowl, whisk eggs and milk until combined and slightly frothy.

Pour egg mixture over bread and vegetable mixture. Toss or press gently to moisten all bread cubes. Let sit 20-30 minutes or up to 1 hour to fully absorb liquid.

Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Transfer mixture to baking dish, spreading evenly. Sprinkle extra Parmesan on top for crispy crust. Bake 35-45 minutes until set and golden brown. Knife inserted should come out clean.

Remove from oven and let rest 5-10 minutes before serving to allow pudding to set further.