Melt 3 tablespoons coconut oil in a large skillet over medium-high heat. Add the onion and apple, and cook 3 minutes, stirring occasionally. Turn the heat down to medium-low, add the garlic, and cook 1 minute more, stirring constantly. Turn off the heat and cool completely.
Combine the cooled apple/onion mixture, ground chicken breast, thyme, salt, sage, fennel seeds, black pepper, nutmeg, and vinegar in a large bowl. Just mix until the apples and onion are incorporated, being careful not to over-mix.
Scoop the sausage mixture into 3-tablespoon-sized balls (I use an ice cream scoop for this), and then flatten the balls into 2 1/2 to 3-inch circles. (Like making mini hamburgers.)
To cook on the stovetop, heat a large skillet or griddle (I use my cast iron griddle, but a nonstick surface will also work) over medium to slightly medium-high heat. Add a little bit of coconut oil and spread it around to coat the skillet or griddle. Add the sausages and cook until they’re golden on both sides and fully cooked inside, about 3 to 4 minutes per side, flipping once. You can work in batches if they don’t all fit at once. (Don’t press down on the sausages as they cook because this will release the juices that keep them moist.)
To cook in the oven, preheat the oven to 425F. Line a large baking sheet with parchment paper and grease it with a little coconut oil. Line the sausages up on the prepared baking sheet, and bake until they’re fully cooked, about 5 to 6 minutes per side, flipping once. (Note that if they’re cooked in the oven, they won’t be golden on the outside unless you run them under the broiler.)
Serve the sausage with eggs or vegetable hash if desired.