Heat the olive oil in a large Dutch oven or deep skillet over medium-high heat. Add the sausage and cook, breaking it into bite-sized pieces, until browned and cooked through, about 6–8 minutes. Transfer to a plate and set aside.

Reduce the heat to medium and melt 1 tablespoon of the butter. Add the leek with a pinch of salt and cook, stirring occasionally, until softened and jammy, about 8–10 minutes. Stir in the red pepper flakes, if using, and cook for 30 seconds.

Add the acini di pepe and cook, stirring constantly, for 1–2 minutes to lightly toast.

Pour in the white wine to deglaze the pot, scraping up any browned bits from the bottom. Let it simmer until reduced by about half, about 1–2 minutes.

Pour in the chicken broth and bring to a gentle simmer over medium-high heat. Reduce the heat to medium-low and simmer, stirring frequently, until the pastina is tender and creamy, about 10 minutes.

During the last 2 minutes of cooking, stir in the grated garlic and the crème fraîche. Let it simmer until slightly thickened.

Remove from the heat and stir in the remaining 1 tablespoon butter, Dijon, Parmesan, lemon zest, if using, and the chives until smooth and creamy. Taste and season with additional salt and plenty of freshly cracked black pepper as needed.

If the pastina has thickened more than you’d like, stir in a splash of chicken broth until it reaches your desired consistency. Serve immediately with extra Parmesan, chives, and freshly cracked black pepper.