Prep the pan and oven: Heat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease lightly.

Make the crumble: In a small bowl, mix 3/4 cup all-purpose flour, 1/3 cup light brown sugar, 1 teaspoon ground cinnamon, and a pinch of salt. Add 6 tablespoons melted unsalted butter and stir with a fork until clumps form. Set aside.

Prep the apples: Peel (optional), core, and dice 2 medium apples into small cubes (about 1/4 inch). Toss with 1 teaspoon lemon juice if you like. Pat dry with a paper towel so extra moisture doesn’t weigh down the batter.

Whisk dry ingredients: In a large bowl, whisk 1 3/4 cups all-purpose flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 1 to 1 1/2 teaspoons ground cinnamon, and a pinch of nutmeg if using.

Mix wet ingredients: In a separate bowl or large measuring cup, whisk 2/3 cup granulated sugar, 1/2 cup neutral oil, 2 large eggs, 3/4 cup Greek yogurt (or sour cream), and 2 teaspoons vanilla until smooth.

Combine wet and dry: Pour wet ingredients into the dry. Stir gently with a spatula until just combined. The batter should be thick and slightly lumpy—don’t overmix.

Fold in apples: Add the diced apples and fold 3–4 times to distribute. If batter seems extremely thick, add 1–2 tablespoons milk to loosen slightly.

Fill the cups: Divide the batter among the 12 muffin cups, filling each about 3/4 full. Sprinkle a generous layer of crumble over each muffin, pressing very lightly so it adheres. For extra sparkle, add a pinch of coarse sugar on top.

Bake: Bake 18–22 minutes, or until the tops are golden and a toothpick inserted into the center comes out with just a few moist crumbs (no wet batter). Rotate the pan once if your oven has hot spots.

Cool: Let muffins rest in the pan for 5 minutes, then move to a wire rack to cool. Enjoy warm or at room temperature.