Step 1Mix together the oil, soy sauce, ginger and garlic. Cut the tenderloins or breast meat into 1-inch cubes and marinate in the soy sauce mixture for 20 minutes.
Step 2Cut the onion in three-fourths inch pieces. Thread the chicken and onion pieces on the skewers, alternating between them. You should have 3 pieces of chicken and 2 pieces of onion per skewer. Season the skewers lightly with salt and pepper. Heat a grill or grill pan.
Step 3Wrap the tips of the skewers with aluminum foil to keep them from burning. Grill the skewers over medium heat about 5 minutes per side, 10 to 12 minutes total.
Step 4When the chicken is done, remove it from the heat. Put a small dab of umeboshi paste on each piece of chicken. Tear the shiso leaves into small pieces and place a piece of shiso on top of each dab of umeboshi paste.