1 9-ounce bag fresh spinach leaves (or 1 big bunch, rinsed, most stems removed, and leaves cut roughly in half)
5 tablespoons olive oil, divided (I used 3-4 T)
1 medium red onion, halved, cut into 1/3-inch-thick wedges with some core attached
1 7-ounce package feta cheese, coarsely crumbled
2 tablespoons Sherry wine vinegar (didn't have; used pomegranate red wine vinegar)