In a large bowl mix the sauce, using a whisk to thoroughly combine all ingredients until smooth. Add the seasoned mahi mahi to sauce and toss to evenly coat, then set aside in refrigerator until needed.
Wipe down cutting board before slicing red onion into thin strips. Next, add olive oil to preheated skillet over medium-high heat. Sauté onions until lightly caramelized, ~10 minutes. Once cooked transfer to a paper towel lined bowl to cool.
When adding marinated mahi mahi, do not feel obligated to add all of the sauce to the skillet. You can simply spoon out the pieces into the skillet. Be sure that you do not cross contaminate your foods on your cutting board. I highly recommend these flexible cutting boards You get one for fish, vegetables, cheese, meat, carving and poultry. Keeping your kitchen safe is a top priority for a healthy family.