MAKE THE CREAMED CHICKEN:
While the biscuits are baking, melt butter over medium heat. Add flour, pepper, and ground mustard and use whisk to cook for 1-2 minutes over medium heat until it smells nutty and there is just a hint of it turning light brown.
Slowly add milk, stirring constantly until it thickens, 4-5 minutes.
Add chicken, peas, and drained pimientos and cook over medium heat until peas are softened and chicken hot - about 10 minutes.
Add cheese; once melted it is ready to serve over biscuits.