Spinach-grapefruit Salad

• 2 pink grapefruit
• 2 oranges
• 1 head radicchio
• 1/2 onion onion rings (thin)
• 60 gr spinach (fresh)
• 30 ml lemon juice
• 30 ml vegetable oil
• 4 gr sugar
• 1 gr pepper (red)
• 1 gr paprika