Rinse the quinoa under cold water using a fine-mesh strainer to remove any bitterness.
In a small saucepan, combine the quinoa and water, bring to a boil, then reduce heat to low, cover, and simmer for 12 to 15 minutes until liquid is absorbed.
While the quinoa cooks, heat the olive oil in a large skillet over medium-high heat.
Add the ground beef to the skillet, seasoning with sea salt and black pepper, and cook until browned, breaking it into small crumbles.
Stir in the diced red onion and red bell pepper, sautéing for 3 to 4 minutes until the vegetables begin to soften.
Add the minced garlic and red pepper flakes, cooking for another 60 seconds until fragrant.
Pour the tamari over the beef mixture and stir well to incorporate the flavors.
Add the baby spinach to the skillet and toss gently for 1 minute until the leaves are just wilted.
Fluff the cooked quinoa with a fork and divide it into bowls.
Top the quinoa with the beef and vegetable mixture and serve immediately.