In a small sauté pan, heat and melt butter. When butter is melted, add in the flour and whisk until blended.
Add in the milk and chicken stock. Bring to a rolling boil and stir frequently. Keep at this heat for 5-7 minutes.
Turn heat off and add in cheese. Stir to blend and melt. Add in salt and pepper to taste
In a 3-4 quart pot, fill the water 3/4 of the way and heat on high. Let water boil and add salt.
Add in the orzo and broccoli and cook together until al dente (around 8-9 minutes). Drain in a strainer in the sink and add back into the pot.
Layer in the cheese sauce and stir until fully coated. Add in 1/2 cup more of cheddar and mix. Serve immediately.