Rustic Potato and Fennel Galette

Name: Rustic Potato and Fennel Galette
Prep time: 10 min
Total time: 1 min
Ingredients: 1 1/4 c. all-purpose flour; 1 tbsp. Thyme leaves; 1/2 tsp. Kosher salt; 1/2 c. cold unsalted butter, cut up (1 stick); 1 tbsp. distilled white vinegar; 1 tbsp. ice-cold water, plus 1-2 teaspoon to add if necessary; 1 small bulb fennel, very thinly sliced; 1 Yukon gold potato, very thinly sliced; 1/2 small red onion, very thinly sliced; 2 tbsp. olive oil; pepper; Kosher salt; 3 oz. shredded Gruyère cheese; 1 egg, beaten