Once melted, add the finely chopped onion and cook, stirring occasional, until softened.
Stir in the garlic and cook just until fragrant, 30 seconds to 1 minute.
Stir in the dried orzo to coat in the butter and toast lightly.
Pour in the chicken stock and bring to a boil. Reduce to a simmer, cover, and cook, stirring often to keep the orzo from sticking to the bottom, until the orzo is tender and most of the stock and been absorbed, 10-15 minutes.
Stir in the heavy cream and remove from heat.
Once the liquid stops bubbling, fold in the shredded Cheddar and two thirds of the grated Parmesan cheese until melted and a creamy sauce coats the orzo.
Season to taste with salt and pepper.
Serve immediately with fresh parsley or basil if desired and a sprinkling of the remaining Parmesan.