To make the pickled onions: Combine the grapefruit juice, orange juice, lime juice, and vinegar in a small saucepan over medium heat. Bring to a boil, turn off the heat, and add the red onion. Let cool in the pan. To store what you don't need for this recipe, transfer them to a quart-size Mason jar and refrigerate for up to 3 weeks.
To make the filling: Pour the olive oil into a large skillet over medium heat. When it shimmers, add the poblanos, onion, and garlic and cook, stirring frequently, until the vegetables start to soften, about 4 minutes. Sprinkle in the cumin, cinnamon, paprika, salt, and pepper and cook for another minute or two, until all the spices are very fragrant. Stir in the beans, reduce the heat to medium-low, and cook just until warmed through. Taste and add more salt and pepper if needed. Turn off the heat and cover to keep warm.
Warm the tortillas in a dry skillet over medium-high heat for a few seconds on each side, then transfer as they're heated to a packet of foil.
To assemble the tacos, lay out the tortillas and top each with some of the bean-poblano mixture. Top each with a spoonful of the salsa, feta, pumpkin seeds, and a few slices of pickled onions and serve hot.
