Preheat your oven to 450F or 425F convection. Line a baking sheet with parchment paper. Add your potatoes, oil, rosemary (if using), and salt and toss to coat. Bake for 15-20 minutes, or until potatoes are crispy and browned. While that roasts, let’s cook the salmon!
Pat the skin and flesh of the salmon filets dry with a paper towel. Sprinkle salt to coat the salmon flesh of each filet and use your fingers to rub it around (about ¼ teaspoon salt per fillet, depending on the size). Flip them over and repeat with the skin side, making sure to press the salt into the skin so that it sticks. This will help form a crust and get your salmon skin crispy. Flip back over and sprinkle the spice mix all over the salmon flesh and use your fingers to rub it all around the salmon flesh (not the skin).
Pat your salmon skin dry once more right before adding to the skillet (dry skin = crispy skin). In a large non-stick skillet, add the oil and the salmon filets, skin-side-down, rubbing the skin in the oil to coat. Turn the heat on to medium-high, once you start to hear the salmon sizzle, you want to press down on each salmon (with a spatula or tongs) for a few seconds so the whole skin is pushed as flat against the skillet as possible. This is important to do for the first few minutes to make sure the entire skin gets crispy.
Reduce the heat to medium and let the salmon cook and continue to crisp up that skin until it’s mostly cooked through to the top, about 5-7 minutes (depending on thickness), pressing down on the salmon occasionally. You’re looking for the salmon to turn from translucent to opaque 3/4 of the way up the side. Then flip the salmon over and cook the flesh side for about 1 minute, or until the salmon is cooked through (you can tell it’s cooked through if it is flaking easily and the salmon looks opaque). Flip the salmon over again with the skin-side down and cook for a minute or so more just to ensure crispiness!
Serve the salmon skin-side-up with a lemon wedge, roasted potatoes, and a simple salad.