Start by rinsing your quinoa in a fine mesh strainer under cold running water for about a minute. This removes saponins, the natural coating that can make quinoa taste bitter or soapy. Set it aside to drain while you prep the rest of your ingredients, since a dry start helps it toast properly later.
Heat 1 tablespoon of olive oil in your skillet over medium-high heat. Pat the shrimp dry, season with salt, pepper, and paprika, then sear for about 1-2 minutes per side until just pink. Remove them to a plate right away, because overcooked shrimp turn rubbery fast.
In the same pan, add the remaining olive oil along with the diced onion. Cook for 2-3 minutes until it softens, then stir in the garlic and red pepper flakes for about 30 seconds. This is where the base flavor of the whole dish really builds, so don't rush this step.
Add the rinsed quinoa directly to the pan and stir it around for a minute to lightly toast it in the oil. Pour in the broth, lemon juice, and half the lemon zest, then bring everything to a gentle boil. Cover, reduce the heat to low, and let it simmer for about 15 minutes.
Once the quinoa has absorbed most of the liquid and looks fluffy, stir in the cherry tomatoes and baby spinach. Cover again for 2 minutes just until the spinach wilts down and the tomatoes soften slightly. The residual heat does most of the work here.
Nestle the cooked shrimp back into the pan, tossing gently so everything warms through together. This only takes a minute or two, since the shrimp are already cooked and you just want them heated, not cooked again.
Sprinkle the remaining lemon zest, feta, and chopped parsley over the top right before serving. I like to add one more small squeeze of fresh lemon juice at the very end for a brighter finish that cuts through the richness of the feta.