Preheat the oven to 425ºF. Line a large baking sheet with parchment paper for easy clean up. Set aside.
Unfold the phyllo: Fair warning – phyllo is incredibly delicate; take extra care as you work with it. Carefully remove the thawed phyllo dough from its packaging & unfold it. Set it atop a damp kitchen towel or damp paper towels on your work station. Cover it with a second damp kitchen towel (or paper towels). Set aside.
Squeeze excess moisture from the spinach: Place the thawed chopped spinach on a kitchen towel (or a few sturdy paper towels). Wring the towel over the sink to squeeze excess water out of the spinach; there will be a lot! Set aside.
Drain & rinse both the marinated artichoke hearts & the olives. Pat dry to remove excess water and set aside.
Cook the onion, kale & aromatics: Add the olive oil to a medium skillet over medium heat. Season with 1 teaspoon kosher salt & cook, stirring occasionally, until the onions begin to soften, 2-3 minutes. Add the kale to the skillet. Continue to cook, stirring occasionally, until the kale is vibrantly green & wilts slightly. Add the garlic, lemon zest, dried oregano, & crushed red pepper (as desired) to the skillet. Stir to combine. Cook for 1-2 minutes more, until fragrant.
Mix the spanakopita filling: Carefully transfer the onion & kale mixture to the bowl of a food processor, along with the squeezed spinach, rinsed artichoke hearts & olives, crumbled feta, pine nuts, & egg. Cover & pulse to combine; the mixture should come together in a homogenous mixture with visible pieces of olive, artichoke heart, pine nuts & cheese. Set aside.
Assemble the triangles: Carefully place a sheet of phyllo dough on the counter. Set a second sheet directly atop the first, creating a 2-ply layer of phyllo. The phyllo may tear a little bit as you handle it, which is okay since it’ll be folded into multiple layers around the spanakopita, though you want to avoid large tears.
Use a small paring knife to slice the phyllo into 4 even strips crosswise. Place 1/4 cup of the spanakopita filling at the bottom end of each phyllo strip. Use a pastry brush to brush olive oil across the length of the rest of each phyllo strip.
Fold the bottom right corner of the phyllo diagonally over the filling to the left edge of the phyllo strip, ensuring that the edges of the dough line up & cover the filling. Gently pat down, as needed, to ensure that the spanakopita filling is evenly dispersed throughout the first fold. Take the bottom left corner & fold it straight up the left edge, ensuring that the edges are aligned.
Fold the bottom left corner of the phyllo diagonally to the right edge of the phyllo strip and gently pat down for even disbursement of the spanakopita. Take the bottom right corner and fold it straight up to the right edge, until the phyllo is completely folded around the spanakopita filling in a nice little triangle. If there is excess phyllo after the final fold, you can trim it off or tuck it around the spanakopita triangle.
Set the folded spanakopita triangle atop the prepared baking sheet & set aside. Repeat with the remaining 3 spanakopita until all of the filling has been used. You should yield about 12 spanakopita triangles.
Place the spanakopita triangles in the oven. Bake for 15-20 minutes, rotating the pan halfway through until the spanakopita triangles are beautifully golden-brown.
Remove the spanakopita triangles from the oven. Allow to cool for at least 5-10 minutes before serving. Serve warm or at room temperature, with your favorite Greek-ish dips & side dishes. Enjoy!