Step 1: Preparation
Bring a large pot of salted water to a boil. Wash and dry produce. Mince chives. Trim and halve Brussels sprouts lengthwise; lay flat, then thinly slice crosswise into shreds. Zest and quarter lemon.
Step 2: Cook Pasta and Toast Panko
Once water is boiling, add spaghetti to pot. Cook, stirring occasionally, until al dente, 9-11 minutes.
Reserve 1½ cups pasta cooking water (2 cups for 4 servings), then drain. (Keep empty pot handy for Step 4.) Meanwhile, melt 1 tablespoon plain butter (2 tablespoons for 4) in a large pan over medium-high heat. Add panko and cook, stirring, until golden and toasted, 2-3 minutes. Stir in half the chives and season with salt and pepper. Turn off heat; transfer to a small bowl. Wipe out pan.
Step 3: Cook Brussels Sprouts
Heat a large drizzle of olive oil in same pan over medium-high heat. Add Brussels sprouts and season with salt and pepper. Cook, stirring occasionally, until bright green and slightly softened, 4-6 minutes. Stir in a pinch of chili flakes if desired; cook for 30 seconds more. Turn off heat.
Step 4: Start Sauce
Once spaghetti is drained, heat empty pot over medium heat. Add cheese roux and 1 cup reserved pasta cooking water (1½ cups for 4 servings); whisk to combine. Cook, whisking, until slightly thickened, 2-3 minutes. Reduce heat to medium low and whisk in cream cheese until melted and combined.
Step 5: Finish Sauce and Pasta
Add drained spaghetti, Brussels sprouts, garlic herb butter, Parmesan, half the lemon zest, remaining chives, and a big squeeze of lemon juice to pot with sauce. Cook, tossing, until butter has melted and pasta is coated in a creamy sauce, about 1 minute. Tip: If needed, stir in more reserved pasta cooking water a splash at a time. Taste and season with salt and pepper. Add more lemon juice if desired.
Step 6: Serve
Divide pasta between bowls. Top with panko and remaining lemon zest to taste. Sprinkle with remaining chili flakes if desired. Serve with any remaining lemon wedges on the side.
