Pre-heat oven to 400 degrees.
Heat a 10-inch cast iron skillet over medium heat and add olive oil. Once oil is shimmering, add onion and cook for 5-7 minutes, until softened and turning brown.
Add garlic and red pepper flakes and cook for about 30 seconds, until fragrant.
Add ground lamb to skillet and cook, breaking up with a wooden spoon, until no longer pink. Drain any excess oil.
Stir crushed tomatoes into lamb mixture, along with ¼ tsp salt, ¼ tsp pepper, and oregano, and bring to a simmer. Cook for about 10 minutes, until sauce begins to thicken. Stir in kalamata olives and baby spinach and stir until spinach begins to wilt, about one minute. Remove meat mixture to large bowl.
In a separate medium bowl, combine ricotta cheese, feta cheese, egg yolk, ¼ tsp salt, and ¼ tsp pepper.
Put about 1 cup of lamb mixture on the bottom of the skillet. Shingle 6 noodle halves around the edge of the skillet. Place dollops of the ricotta/feta mixture on the noodles, using half the mixture.
Top with half of the remaining meat sauce and half the mozzarella cheese and shingle the remaining 6 noodle halves around the skillet. Dollop with the remaining ricotta/feta mixture. Layer on the remaining meat sauce and remaining mozzarella.
Bake for 22-25 minutes, until lasagna is bubbling around the edges and the cheese is golden.
Let cool for a few minutes before sprinkling with basil and serving.