Preheat oven to 400°F; position rack in lower third of oven. Combine potatoes, 2 tablespoons salt, and enough cold water to cover by 1-inch in a large saucepan; bring to a boil over high, undisturbed. Reduce heat to medium; cook, undisturbed, until potatoes are fork-tender, about 25 to 30 minutes. Drain well and pat dry.

Generously brush a 10-inch cast-iron skillet evenly with 2 tablespoons oil. Place drained potatoes in an even layer in skillet; using a flat bottom measuring cup, smash potatoes firmly into the bottom, then press over bottom and about 1 ½-inch up sides of skillet. Use a spoon or small offset spatula to evenly spread potatoes, smoothing and patching any holes or thin spots as needed. Brush potato crust evenly with 1 tablespoon oil and sprinkle evenly with 1 teaspoon salt. Bake potato crust until edges are golden brown and slightly crisp and center is lightly golden, 35 to 40 minutes.

While crust bakes, heat remaining 1 tablespoon oil in a separate medium skillet over medium. Add onions and cook, stirring frequently until very softened and tender, 5 to 7 minutes. Remove from heat and stir in spinach until evenly combined. Set aside.

Remove skillet from oven. Sprinkle half of the feta (about ½ cup) in an even layer over bottom of potato crust. Top evenly with spinach mixture followed by remaining half of feta.

Whisk together eggs, cream, dill, mint, lemon zest, garlic powder, and remaining 1 ½ teaspoons salt in a large bowl until well combined, about 30 seconds. Pour egg mixture over filling in skillet, spreading into an even layer, if needed. Carefully transfer to oven using kitchen mitts. Bake until filling is set and a paring knife inserted in center comes out clean, 20 to 25 minutes, loosely covering edges with aluminum foil to prevent overbrowning if needed. Let cool slightly on a wire rack before serving, about 20 minutes. Serve garnished with more dill and mint.