Prepare a cheese mold by lightly oiling a smooth glass container.
Pour all ingredients (oat milk, agar-agar, salt, garlic powder, and lemon juice) into a medium-sized saucepan. (Add tapioca starch at this time if you want cheese that will melt and stretch.)
Stir the ingredients, then turn the heat to medium and bring to a slow boil.
Cook your cheese sauce over low heat for 6 minutes, stirring frequently. (It needs to boil for 6 minutes to fully melt and activate the agar-agar.)
Pour the cheese into the chosen mold that has been sprayed with oil
Cool for about 15 minutes on the countertop, then refrigerate, uncovered, for 2 more hours, until it cools and sets completely.
Notes
Stir the cheese sauce very frequently. Don't walk away while cooking.
Keep in an airtight container in the fridge for up to 5 days or in the freezer for up to 3 months.
Measure the ingredients carefully! Slight changes in the amounts of liquid, agar-agar, or tapioca starch can significantly affect the texture of your cheese!
Simmer the cheddar cheese for 6 minutes to melt the agar-agar and allow it to bind completely with the ingredients.
Add 2 tablespoons of tapioca starch if you are using this cheese in anything hot, like a grilled cheese, where you want it to melt.