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1
In a medium saucepan over medium high heat, bring milk, onion, garlic to simmer; lower heat and cook for about 8 minutes on very low heat. Add salt and pepper to taste. Set aside.
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2
In a medium saucepan, melt butter over medium heat. Slowly whisk in flour and cook for over low heat until fragrant and slightly thickened, about 4 minutes. After removing onion and garlic, slowly whisk in hot milk. Let cook, stirring occasionally, until thickened (but not too thick), about 5 minutes. Stir in 1 1/2 tablespoons of the mustard; add more to taste (you want it to be subtle, not loud like a hot dog at the ball park). Cover the sauce with plastic wrap and set aside.
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3
Preheat oven to 375. Blanch Brussels sprouts until barely tender, about 3 minutes. Drain; set aside. In a medium skillet, over medium high heat, cook the pancetta to render fat. Toss in Brussels sprouts and stir to coat in fat, and brown slightly, 5-7 minutes. Sprinkle with the lemon juice; add a little salt and pepper.
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4
Remove the crusts from the rye bread (I used pre-sliced because no market seems to care about rye bread in Chicago; if you can buy a really good loaf, slice it to a similar sandwich thickness). Cut each slice into four pieces crosswise.
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5
Lightly butter a casserole that will hold the sprouts in a single layer. Spread Brussels sprouts on bottom, sprinkle with 3/4 of the cheese. Layer bread in single row to cover. Press the bread down with your hands a bit so that it is all snuggly.
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6
Whisk the half and half into the cooled Bechamel to thin it slightly (you can use whole milk here). Pour over the casserole, spreading to coat the bread completely if possible. Sprinkle with remaining cheese; season top with about 1/4 teaspoon freshly ground black pepper. Let the casserole sit for at least 15 minutes before placing in oven. Cook until golden brown on top and bubbly, 20-25 minutes. Serve very warm but not piping hot.
