Cut chicken breasts (kitchen scissors work best) into 1-inch cubes.
Mix lemon juice, soy sauce, mustard, oil and pepper.
Put marinade and chicken cubes in a 1-gallon bag, and store in the freezer.
To prepare for serving, thaw chicken and remove from marinade.
Warm marinade in a small saucepan.
Place cubes about an inch apart on broiler pan treated with nonstick spray.
Broil 4 to 5 inches from heat for 7 minutes, brushing with marinade once.
Turn chicken cubes, and broil another 4 minutes.