While the chicken bakes, pour the oil out of the skillet and wipe it out with a paper towel. Return the skillet to medium high heat and add 1 Tbsp avocado oil.
Add the garlic and cook for 20 seconds, then add the onion and cook for 3 to 5 minutes, until translucent.
Add the bell pepper and cook for 3 to 5 minutes.
Whisk the sauce again to make sure it is well mixed, then pour it into the skillet. Bring it to a light simmer.
In a small bowl, stir the cornstarch into the water to make a slurry. Once the sauce is simmering, stir in the slurry. Keep stirring and cook for about 2 minutes, until the sauce thickens.
Stir in the pineapple just to warm it through, then take the skillet off the heat.
When the chicken is done, add it to the skillet and toss to coat it in the sauce. Serve right away over rice.