Rinse the quinoa in a strainer under cold water until the water runs clear.
Bring the quinoa and the 8 cups of water to a boil in a large saucepan over medium-high heat. Reduce the heat to medium low and simmer until the white outer casings on the quinoa have popped, revealing translucent little beads, about 10 to 15 minutes.
Strain the quinoa through a fine-mesh strainer set over a large heatproof bowl. Measure the quinoa cooking liquid and add water as needed to make 6 cups; set the quinoa and cooking liquid aside.
Heat the olive oil in a large, clean saucepan over medium-high heat until shimmering. Add the garlic and jalapeño and cook until fragrant, about 30 seconds. Add the potatoes, cumin, and measured salt and cook, stirring occasionally, until the potatoes begin to soften, adjusting the heat as necessary so that the garlic doesn’t brown, about 3 minutes.
Add the reserved cooking liquid and half of the scallions and simmer until the potatoes are tender, about 12 minutes.
Add the cooked quinoa, spinach, and remaining scallions and simmer until the spinach just begins to wilt, about 3 minutes. Remove the pan from the heat and stir in the feta and cilantro. Taste and season with salt and pepper as needed. Top with the chopped egg and serve.