Bring water, quinoa, and 1/2 teaspoon sea salt to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until quinoa is tender and water has been absorbed, 15 to 20 minutes. Spread quinoa on a baking sheet and cool, about 30 minutes.

Mix quinoa, tomatoes, feta cheese, sunflower seeds, parsley, olives, and shallot together in a large bowl.

Whisk olive oil, lemon juice, mustard, garlic, 1/4 teaspoon sea salt, and pepper together in a bowl; pour over quinoa mixture and toss to coat.