In a large skillet, sauté the garlic and onions in 2 Tbs butter over medium heat until transluscent. Stir in the wine and lemon juice, and increase heat and allow to simmer.
When reduced by half, reduce heat to low and slowly stir in the cold butter, one slice at a time. Stir in salt, pepper, basil, and tomatoes.
Spoon mixture over cooked chicken breasts. Top with a thick slice of cheese.