In a large saucepan or a high-sided skillet, melt the butter over medium heat. Add the diced onion and cook until soft and translucent. Stir in the garlic and cook for about 30 seconds, just until fragrant.

Sprinkle in the flour and cook for 30–60 seconds, stirring constantly. Slowly whisk in the half-and-half until smooth.

Add the frozen peas and bring to a gentle simmer, stirring often. Let the mixture cook until the cream thickens, about 10–15 minutes. Keep the heat moderate — you want a gentle simmer, not a boil.

Stir in the parmesan until fully melted. Season with salt and pepper to taste, then finish with a bit of lemon zest and a squeeze of lemon juice.

Take a moment to taste and adjust the seasoning or add additional salt, then serve right away.