- Gluten-Free Crust:
- 3 cups gluten-free all-purpose baking flour (such as Bob's Red Mill®)
- 2 teaspoons coarse kosher salt, divided
- 1 teaspoon instant dry yeast
- 1 cup water, or more as needed
- 2 tablespoons olive oil
- Toppings:
- 2 teaspoons olive oil, or more to taste
- 1 teaspoon balsamic vinegar, or more to taste
- 1 small red onion, thinly sliced
- 4 cloves garlic, minced, or more to taste
- 1 large Fuji apple, chopped
- 3 heads radicchio, shredded, or more to taste
- salt and ground black pepper to taste
- 1/4 cup chopped fresh parsley, or to taste
- 2 teaspoons honey, or more to taste
- Combine flour, kosher salt, and yeast in a food processor. Pour in water and olive oil, with the food processor running, until the mixture forms a sticky ball of dough. Add more water if needed.
- Turn dough onto a floured surface and knead until it forms a smooth, round ball, about 10 seconds. Place dough in a greased bowl. Cover with plastic wrap and let rise in a warm, draft-free area until it doubles in size, 1 to 2 hours.
- Heat 1 teaspoon olive oil and balsamic vinegar in a skillet over medium heat; cook and stir onion and garlic until softened and slightly browned, 5 to 8 minutes. Add apples; cook and stir until slightly softened, 2 to 3 minutes.
- Toss radicchio with remaining 1 teaspoon olive oil, salt, and pepper together in a large bowl.
- Preheat oven to 450 degrees F (230 degrees C).
- Spread dough onto a pizza stone or greased baking sheet.
- Bake in the preheated oven until pale, 5 to 7 minutes. Spread a thin layer of onion mixture onto crust and top with radicchio. Bake in the oven for 5 minutes more. Turn on the oven's broiler and broil until edges begin to brown, 1 to 2 minutes.
- Remove flatbread from oven and garnish with parsley. Top with a drizzle of honey.