Heat oven to 400°F. Pierce sweet potatoes; roast on a rimmed sheet 55 to 65 minutes until fully tender. Cool 10 minutes; remove skins and mash in a large bowl. Reduce oven to 375°F.
Melt 2 tablespoons butter over medium. Cook garlic 30 seconds; add squeezed spinach and stir 3 minutes until moisture evaporates. Fold into mash; cool until warm, not steaming.
Grease a 2-quart dish with teaspoon butter. Fold yogurt, egg, feta, salt, and pepper into mash just until combined; spread in dish.
Toss panko and pumpkin seeds with oil; scatter thinly over filling. Bake 25 to 30 minutes until top golden and center reaches 165°F.
Rest 10 minutes to settle filling. Serve with a broad spoon, keeping topping on each portion.