Heat the oil in a skillet or saucepan over medium-low heat. Add the onion and saute until softened, about 5 minutes. Add the garlic and saute 1 minute more.
Whisk in the peanut butter, coconut milk, chili sauce, soy sauce and brown sugar until relatively smooth. Bring the mixture to a simmer, reduce the heat to low and let simmer for 10 minutes, uncovered, stirring occasionally.
If it thickens too much for your needs, whisk in a little water to your desired consistency.
Notes
Coconut Milk: This is regular coconut milk that comes in cans or little aseptic packages, not coconut milk beverage. It should be quite rich. For a thinner sauce, you can use light canned coconut milk. Using coconut milk beverage will produce a runnier sauce.
Recipe by Go Dairy Free at https://www.godairyfree.org/recipes/sauces/satay-sauce-vegan-gluten-free