Slice one onion horizontally nearly in half (leaving onion connected). Repeat vertically, forming a cross. Slice twice more, forming an X so that onion has eight sections. Place onions on foil on a baking sheet.
Combine oil and vinegar. Drizzle evenly over onions. Season with salt. Bake at 400 degrees for 25-30 minutes, or until onions are tender and flower shaped.
Optional: garnish with edible petals and balsamic vinaigrette.
One onion per serving.