Preheat oven to 400 F.
First, prepare quinoa by placing quinoa and water with salt into a medium-sized pot. Bring quinoa to a boil over medium-high heat and then turn the burner down to low, cover, and let simmer for 15-18 minutes or until water has absorbed.
In the meantime pour the olive oil in a baking dish. Scatter the garlic cloves and onion. Add the tomatoes and lemon slices and toss. Scoot to one side.
Pat the fish dry and place in the baking dish, and using tongs, turn to coat each side of the fish with oil. Spread out the tomato garlic mixture and nestle in the fish. Tomatoes on the sides, lemons underneath. Season all generoulsy with salt, pepper and chili flakes.
Bake for ten minutes. Give the pan a good shake,jostling the tomatoes a bit. Scatter with lemon zest. Bake 5 more minutes.
When done, add the chopped dill, tossing it with the warm tomatoes. Serve with quinoa.