In a small saucepan, combine quinoa, salt, and water. Bring to a full boil. Cover and reduce heat to low; cook for 10 minutes. Remove from heat. Let sit for 5 minutes; then fluff with a fork.

Meanwhile, heat olive oil over medium-high heat in a large skillet. Add onions and peppers and sauté until soft, about 5 minutes. Add spinach and beans and continue cooking until spinach is wilted, about 5 minutes. Add garlic, red pepper flakes, and salt and pepper to taste. Stir to combine.

In a large bowl, combine cooked quinoa with spinach mixture. Serve immediately.