Pat the salmon fillet dry with a paper towel and season both sides evenly with the sea salt and black pepper.
In a small glass bowl, whisk together the fresh lemon juice, lemon zest, and Dijon mustard until the mixture is well combined.
Heat the extra virgin olive oil in a medium non-stick skillet over medium-high heat until the oil is shimmering.
Carefully place the salmon fillet in the pan, skin-side up, and sear for 4 minutes without moving it to develop a golden-brown crust.
Flip the fillet over and pour the lemon-mustard sauce into the pan, using a spoon to baste the top of the fish with the liquid.
Cook for another 3 minutes until the salmon is just opaque and the sauce has slightly thickened.
Remove the salmon from the skillet and set it aside on a plate to rest.
Immediately add the minced garlic and baby spinach to the hot skillet, tossing for 60 seconds until the spinach is just wilted.
Plate the garlicky spinach and place the salmon fillet on top, drizzling any remaining tangy pan sauce over the fish.