In a pot with a lid, bring 2 cups water to a boil over high heat. Add the quinoa, turn the heat to low, and simmer, covered, until all of the liquid is absorbed, 15 to 20 minutes. Fluff with a fork and let cool.

Preheat the oven to 450°F. Line a rimmed baking sheet with parchment paper. Season each salmon fillet with 1/4 teaspoon salt and place them 1 inch apart, skin-side down, on the baking sheet. Roast for 7 to 8 minutes, depending on thickness. Immediately transfer to a plate to cool (they will continue cooking a bit).

In a blender, blend the sunflower seeds and vinegar until smooth. Add the tamari and oil and blend to combine.

In a salad bowl, toss together the quinoa, tomatoes, onion, spinach, cucumber, and dressing. Divide among four dinner plates, top with the salmon, add a sprinkle of pepper, and serve.