Bring 1 cup of water to a boil in a saucepan. Rinse the quinoa under cold water, drain it well, and stir it into the pot with the salt. Cover, lower the heat, and cook for 15 minutes.
Turn off the heat and leave the lid on for 5 minutes. This quiet rest lets the grains finish steaming and puff up. Fluff with a fork afterward so every grain stays light and separate.
While the quinoa cooks, add the shrimp to a medium bowl. Toss in the garlic, olive oil, paprika, salt, onion powder, thyme, oregano, black pepper, and cayenne. Mix well until every shrimp wears a thin, even coat of spice.
Place a large heavy non-stick pan over medium-high heat for about 2 minutes. A properly hot pan is the secret to a good char. Add a light drizzle of oil only if your pan needs it.
Cook the shrimp in batches, about 2 minutes per side. You want them charred on the outside and cooked through, with a pink, curled shape. Do not crowd the pan, or the shrimp will steam instead of blacken.
Chop the romaine, dice the tomato and onion, and slice the avocado. Keep the avocado for last so it stays bright and green. A quick squeeze of lime also helps stop browning.
Divide the lettuce between two large bowls. Top each one with half the quinoa, shrimp, avocado, tomato, and onion. Arrange each ingredient in its own section for a neat, colorful look.
Squeeze fresh lime juice over each bowl. You can also drizzle on sour cream or your favorite creamy dressing. Serve right away while the shrimp is still warm and the greens are crisp