Open Faced Healthy Tuna Melt

• 1 onion (red finely chopped)
• 1/2 tomato (diced)
• 120 gr feta cheese crumbled
• 120 ml olive oil
• 34 gr black olives (chopped)
• 30 ml wine vinegar (red)
• 3 gr parsley
• 1 package flaked tuna drain (white)