Pour oil into a small, oven-safe skillet. Place the spinach and garlic in the skillet, season with a pinch of salt and pepper, and mix well so that everything is evenly coated.
Place the salmon on 1 side of the pan, then form a well on the other side and crack an egg, making sure it stays intact.
Season the tops of the salmon and the egg with another pinch of salt and pepper.
Bake for 20-25 minutes, until the salmon reaches an internal temperature of 140°F (60°C). (The temperature will continue to rise once it rests out of the oven.)
Squeeze a bit of fresh lemon juice over the salmon fillet for more flavor and let the salmon rest for 5 minutes before serving.
NOTE: If you want a runny egg yolk, follow steps 1-3, then place the skillet in the preheated oven. Halfway through the cooking process (about 10 minutes in), crack a cold egg into the well of the spinach and replace the skillet in the oven for another 10-15 minutes. The salmon will be cooked through, the egg white will show color, and the yolk will be gloriously runny.