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Melt 6 tablespoons of butter in a large skillet over medium high heat. Add 6 tablespoons of flour and whisk until well-blended, thick, and no visible flour remains.
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Add 2 cups of half and half to the cooked flour and butter mixture. Stir until the sauce starts to simmer around the edges. Reduce heat to medium. Add thyme, salt and pepper. Continue to stir and simmer until thick.
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Remove pan from heat and stir in grated swiss cheese until melted. Taste! More salt or pepper?
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Pour the cream sauce over the celery and onion mixture.