Oh, this is sooooo soooo good! Definitely company worthy & makes a beautiful presentation. This is one of my mom's favorite entrees for me to make for her. Obviously adaptable to what you like and have on hand. Enjoy!
Preheat oven to 400 degrees. Spray flat baking sheet or foil on a baking sheet with cooking spray & put in oven until you are ready to go.
Heat some olive or canola oil in sautee pan (enough to coat bottom lightly). Add the diced onion & toss for a few minutes. Add spinach and some lemon zest, stirring until spinach is wilted (a few minutes). Add toasted walnuts, basil, and roasted pepper and stir. Remove and let cool down. This is your "stuffing."
Make a pocket in each salmon fillet (make sure you get a thick enough salmon to make this easier) without cutting through the back -- if you do cut through the back, don't worry about it. I have, and it turns out fine, just not as pretty. Stuff each filet with as much of the stuffing as it will hold. I use my fingers to do this.
I sprinkled some lemon juice over each filet. Then, brush each filet with dijon mustard.
Mix panko, melted butter, and your choice of seasonings together until the it is a bread crumb topping consistency, adding more butter or panko as needed. Sprinkle this over the salmon.
Bake for about 12 minutes, depending on your oven, until the salmon is flaking. Sprinkle some feta crumbles over the top and garnish with lemon slices. This even tastes delicious cold the next day! Enjoy!