Put potatoes and lemon juice in a bowl, toss gently and let sit for 10 minutes. Drain off excess liquid. Add onion, cream, salt and pepper; mix.
In a medium nonstick frying pan, bring butter to a fast bubble. Add potato mixture and cover. Cook over medium heat until potatoes are brown. Turn potatoes, cover and cook until potatoes are light brown on both sides.
Remove from pan and cut into squares.