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Grease a 9x13 inch casserole dish and preheat oven to 350 degrees.
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Place a large non-stick pan over medium heat and add butter and melt, add onion slices and cook for 1-2 minutes, until just tender.
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Add minced garlic, asparagus, mushrooms, and cook, stirring occasionally, for about 6-8 minutes.
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Season with salt, pepper, paprika, thyme, and lemon zest. Stir to combine.
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In a measuring cup combine cream and flour, whisk to combine.
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Add the flour mixture to the skillet and cook, stirring for 3-4 minutes, or until thickened.
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Reduce heat to low and mix in mayonnaise and cream cheese, stir until fully combined.
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Remove from heat and stir in both kinds of cheese until melted.
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Crush about 1 1/2 cups of french-fried onions and mix them into the asparagus mixture.
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Transfer mixture to prepared casserole dish.
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Sprinkle remaining french-fried onions on top.
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Place in oven and bake for 30-40 minutes or until mixture is bubbly.
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Top with more french-fried onions and serve.