Tomato and cheddar tart with a thyme buttermilk crust

Name: Tomato and cheddar tart with a thyme buttermilk crust
Ingredients: For crust dough; 1/2 cup all-purpose flour (plus more for rolling); 3/4 cup whole-wheat flour; 1/2 teaspoon kosher salt; 1/2 cup cold unsalted butter, one stick; 1 tablespoon fresh thyme leaves, rinsed with woody stems removed; 1/4 cup buttermilk, whole fat (if not available, you can use whole milk yogurt OR 3 Tbsp Greek Yogurt thinned with 1 Tbsp of water); For the filling; 1 cup sharp cheddar cheese, grated; 1/4 cup Parmesan or Pecorino Romano cheese, grated; 1 shallot (or 1/2 of a red onion); 3-4 ripe but firm tomatoes