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Preheat oven to 400F.
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Put pasta on to boil.
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In a very large, cast iron skillet or oven safe lasagna-type dish, sauté the onions in the oil. Add the chicken and cook over medium-high heat until browned.
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Remove onions and chicken from the pan.
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Melt butter in pan, then add flour and whisk together until the flour absorbs the butter. Add the broth, milk, and mustard powder and bring to a simmer, whisking until the mixture thickens, then remove from heat immediately and add kale, stirring to mix. Add salt and pepper as desired.
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Add the chicken and cooked pasta to the pan, tossing to combine all ingredients.
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Layer the Havarti Slices on top of the other ingredients.
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Cook at 400F for 15-20 minutes, until cheese is bubbling, and chicken is cooked through.