On a cutting board, dice cucumber into 1/4-inch pieces - to peel or not to peel is up to you. Seed and dice the tomatoes into 1/4-inch pieces. Combine the cucumber, tomatoes and half of the shallots or red onion in bowl and reserve.
In a small bowl, whisk together mustard, sugar, and white wine vinegar and remaining half of the shallots. Stream in extra-virgin olive oil while continuing to whisk. Stir in dill and season dressing with salt and pepper, to taste.
Season the salmon with seafood seasoning and a little black pepper. Heat a nonstick skillet with a drizzle of extra-virgin olive oil over medium-high heat. Place salmon rounded side down and cook until golden and a little crispy at edges, 3 to 4 minutes. Flip and cook 2 minutes more for a pink center, 4 minutes for opaque fish.
Transfer salmon to dinner or serving plates, top with the cucumber-tomato relish and cover with a liberal amount of dill dressing.