PREHEAT OVEN TO 350°F AND GREASE A 9×13-INCH CASSEROLE DISH WITH BUTTER OR NONSTICK SPRAY.
COOK RICE ACCORDING TO PACKAGE DIRECTIONS. SET ASIDE.
IN A LARGE SKILLET, MELT BUTTER OVER MEDIUM-HIGH HEAT. ADD ONION AND COOK UNTIL TRANSLUCENT, ABOUT 3 MINUTES. WHISK IN FLOUR AND GARLIC POWDER AND COOK 1 MINUTE MORE.
GRADUALLY ADD MILK AND COOK UNTIL BUBBLY AND THICK. STIR IN CHEDDAR CHEESE AND SEASON TO TASTE WITH SALT AND PEPPER.
ADD RICE, TUNA, PEAS, CHEESE SAUCE, AND SOUR CREAM TO CASSEROLE DISH AND STIR TO COMBINE.
COMBINE CRACKER CRUMBS WITH PARMESAN CHEESE AND SPRINKLE OVER THE TOP OF CASSEROLE.
BAKE UNCOVERED UNTIL BUBBLY, ABOUT 20 MINUTES. BROIL ON HIGH UNTIL TOP IS GOLDEN BROWN, IF DESIRED.
LET REST 5 MINUTES BEFORE SERVING. ENJOY!