Preheat oven to 400 degrees. Arrange onion slices in a single layer on rimmed baking sheet; drizzle 1/4 cup olive oil over; turn onions to coat; sprinkle lightly with salt.
Roast until soft and brown on edges, turning every 15 minutes - will take about one hour. Transfer to platter and cool.
When ready to serve, sprinkle cheese over onions. Drizzle with 1/4 cup olive oil, vinegar, and top with toasted almonds. 6-8 servings.
*To keep onion slices intact during roasting, push short wooden skewers (soaked for 30 minutes in water) through each slice.