1. To prepare crust, place butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Combine milk, salt, and egg yolk in a small bowl; stir well with a whisk. Add milk mixture to butter, 1 tablespoon at a time, beating well after each addition. Add flour; beat just until combined. Press mixture into a 4-inch circle on plastic wrap; cover. Chill for 1 hour.

2. Preheat oven to 350°.

3. Unwrap and place chilled dough on a lightly floured surface. Roll dough into a 10-inch circle. Fit dough into a 9-inch pie plate. Freeze 15 minutes. Bake at 350° for 25 minutes or until lightly browned. Cool.

4. To prepare filling, heat oil in a large skillet over medium-high heat. Add onions; sauté 5 minutes or until tender. Add spinach; sauté 2 minutes.

5. Combine 1 cup milk and remaining ingredients in a bowl; stir well with a whisk. Stir in spinach mixture. Pour filling into crust. Bake at 350° for 35 minutes. Cut into 10 wedges.