Västerbotten Cheese and Red Onion Spelt Galette

Name: Västerbotten Cheese and Red Onion Spelt Galette
Prep time: 20 min
Cook time: 35 min
Total time: 55 min
Cuisine: European
Ingredients: 175g (1½ cups) spelt flour (see note 1); 1 tsp salt; 85g (¼ cup + 2 tbsp) butter (cold); 3 tbsp cold water; 30g (2 tbsp) butter; 1 small red onion (finely sliced); 1 clove garlic (crushed); 1 tsp Worcestershire sauce; salt & pepper (to taste); 100g (3½ oz) Greek yoghurt; 110g (4 oz) grated Västerbotten cheese (see note 2); 1 egg yolk (for "washing"); sea salt (to taste)