Fit Pillsbury crust Pyrex plate. Prick fork bottom. Line parchment, fill pie weights. Bake 375°F 15 minutes. Remove weights. Bake 5 minutes gold. Cool rack.
Melt 2 tbsp butter skillet medium. Add onion. Stir wooden spoon 3 minutes soft. Add spinach handfuls. Wilt 2 minutes. Squeeze dry paper towels. Cool.
Whisk eggs bowl 1 minute frothy. Whisk heavy cream, salt, nutmeg 30 seconds smooth. Fold feta, Swiss, spinach-onion mix spatula even.
Pour custard crust slow. Bake 375°F 35 minutes center jiggles slight, top gold. Cool 20 minutes sets. Slice knife wedges.