Melt the butter in a saucepan over medium-high heat, then add the onion and garlic and saute until softened, 1 to 2 minutes.
Add the orzo and stir until thoroughly mixed with the onion and garlic and coated in the butter, a minute or so. Add a third of the broth and stir gently, then bring to the boil. Add the dry mustard, cayenne, salt and pepper and cook until the liquid has been absorbed by the orzo, 3 to 4 minutes, then add another third of the broth. Cook, stirring, until this, too, has been absorbed, another 3 to 4 minutes.
Add the broccoli followed by the remaining broth and the pimientos, then bring to the boil. Cook until the broccoli and orzo are almost cooked, 8 to 10 minutes. Add the cheeses, stirring, and cook until completely melted and smooth and the broccoli and orzo are cooked through, another 1 to 2 minutes.
Taste and adjust the seasonings as needed.
Transfer to serving bowls and garnish with the fresh parsley.