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1
Place spinach on a plate and microwave it for 6 minutes on HIGH to defrost it; place it in a clean kitchen towel and wring it dry; set aside.
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2
While the spinach is defrosting, get a big pot of water on the stove; bring to a boil; then salt the water liberally and cook the shells to al dente.
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3
While the pasta cooks, heat a deep large skillet over medium heat.
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4
Add in the olive oil, then melt the butter into the oil.
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5
When the butter melts, add in the onions and garlic; cook 4-5 minutes until tender.
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6
Sprinkle the flour around the pan and cook for a minute, then whisk in the wine-it will cook off and the mixture will thicken almost immediately.
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7
Whisk in the stock, then whisk in the milk and bring it to a bubble.
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8
Reduce heat slightly; season the sauce with nutmeg, hot sauce, and mustard; season with salt and pepper to taste.
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9
Simmer 2-3 minutes to thicken; then add in the spinach, separating it as you add it to the sauce.
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10
Preheat the broiler.
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11
Back to the sauce, add in the tuna, flaking it as you go; then stir to combine.
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12
Heat the spinach and tuna for a minute or so.
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13
Drain the pasta and toss with the sauce.
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14
Transfer mixture to a casserole dish; cover it with the cheese.
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15
Place the casserole under the broiler for 2 minutes to brown the edges and the cheese.
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16
Top with parsley and serve.
