In a large nonstick skillet, melt butter over medium-high heat. Add onion and sauté until softened and translucent, stirring frequently, about 2-3 minutes. Add garlic and cook for 30 seconds until fragrant.
Stir in the salt, pepper, and a pinch of cayenne. Then whisk in the flour until fully incorporated and no white streaks remain to create a roux, stirring constantly so it doesn't burn.
Slowly pour in the half-n-half and whisk to combine, making sure the roux isn't clumpy.
Add in the peas; cook, stirring frequently for about 4 minutes, or until sauce has thickened and peas are warmed through.
Remove from heat and stir in the Parmesan cheese until melted.
Taste and adjust seasonings, if desired. Serve immediately while hot and still shiny with a little cracked black pepper and fresh chopped parsley.
