Boil potatoes in lightly salted water until tender. Drain.
When potatoes are just cool enough to handle, dump them onto a cutting board and roughly chop them into quarters. Return them to the pot or place them in a bowl.
Dump the mustard, mayonnaise, salt, and pepper over the potatoes. Lightly toss until incorporated.
Add the red onions and the peas. Gently combine (do not squash your peas).
Shake adobo above and under the skin of the chicken. Lightly oil your hands and massage the spice into the chicken. If you have time to do this step ahead of time then do so. If not, no worries.
Place the chicken (skin-side down) on a 400° pre-heated grill. Cover with a brick. Cook until the skin is golden (around 10-15 minutes). If you see flames, just move the chicken to another spot on the grill.
Flip the chicken over, cover once again with the brick (use a very thick mitt). Cook another 10-15 minutes, or until the chicken reaches 165°.
Remove chicken to a plate and allow to rest for a few minutes before cutting.