Thoroughly squeeze any excess water from the thawed spinach, removing as much liquid as possible. Coarsely chop.
Melt 1 tablespoon butter in a large skillet over medium-high heat. Add the garlic and cook until fragrant and golden brown. Add the spinach and nutmeg, sauteing for 1 to 2 minutes. Remove from the heat and mix in the feta, lemon juice, cracked black pepper and egg to thoroughly combine. Allow the filling to cool.
Melt the remaining butter over medium heat in a small saucepan. Cool slightly. Cover the stack of phyllo sheets with a clean damp kitchen towel.
Working with the long side of the phyllo sheet nearest you, take 1 sheet from the stack and brush the entire surface with butter. Top with another phyllo sheet, brushing with more butter. Cut crosswise into five 2 1/2 to 3-inch strips.
Place 2 teaspoons filling in the corner of each strip. Just like you would fold a flag, fold the corner of phyllo over to enclose the filling, forming a triangle, and continue folding the entire strip. Ending with the seam-side down, place the triangles on a large baking sheet and brush the tops with butter.
Follow the same steps with the remaining ingredients.
Space evenly on a sheet tray and bake until golden brown, about 25 minutes. Remove from the oven and transfer to a wire rack to cool slightly.