Heat oil in a small sauce pan over medium heat, and add in the onion. Cook onion until translucent and add in adobo seasoning, tomato paste, rice, salt, and goya sazon con achiote y annatto, until it is all combined. Add in the water and gandules. Bring to a boil, and then lower heat and cover until rice is fully cooked (about 20 minutes). Once rice is cooked, raise heat for 2-3 minutes to crisp rice on the bottom of the pan, and then turn off heat. Serve and enjoy warm!