Brussels Sprouts Casserole

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2 tablespoons extra-virgin olive oil

2 pounds Brussels sprouts, quartered at the root end (or halved if small) 

4 tablespoons unsalted butter 

2 leeks, white and light green parts, halved and sliced 1/4-inch-thick  

Kosher salt and freshly ground black pepper 

2 teaspoons chopped fresh thyme 

3 cloves garlic, sliced 

3 tablespoons all-purpose flour 

2 cups low-sodium chicken broth 

1 cup heavy cream 

1/4 cup grated Parmesan 

2 teaspoons Dijon mustard 

Zest of 1 lemon 

2 cups crispy onions, such as French's