Heat oven to 400°F. Spray 36 mini muffin cups with Crisco® Original No-Stick Cooking Spray. Microwave frozen spinach as directed on box 3 to 4 minutes to thaw. Squeeze dry with paper towels.
In 12-inch nonstick skillet, cook ground beef over medium-high heat 4 to 5 minutes or until no longer pink, stirring to break up large pieces. Stir in onion. Cook 3 to 4 minutes or until tender; drain. Stir in marinara sauce and half of the spinach.
Meanwhile, unroll pie crusts. Using 2 1/4-inch round cookie cutter, cut 18 rounds from each crust, rerolling dough if necessary. Press each round in bottom and up side of muffin cup. Spoon rounded tablespoon meat mixture in each cup.
Microwave mashed potatoes as directed on package 2 to 3 minutes or until warm. In medium bowl, mix potatoes, remaining spinach, cheese, 1/4 teaspoon salt and 1/4 teaspoon pepper until well blended. Top each cup with rounded tablespoon potato mixture.
Bake 20 to 25 minutes or until potatoes are golden brown. Cool in pans 2 minutes. Run knife around edge of cups to loosen. Serve warm.