In a cast iron skillet or a nonstick frying pan, heat up olive oil over medium-high heat. Add prepared red bell pepper, zucchini, and red onion. Cook for 5 minutes or until the vegetables are turning soft but are still crunchy, stirring frequently.
Add mango into the pan, stir to combine. Turn off heat and transfer to a medium bowl. Let cool.
Once the vegetable mango relish cools down, mix lemon vinaigrette and basil into it. Enjoy as a side or serve over fish or pork chops.
If you want a little kick in your relish, add a pinch of red pepper flakes while cooking.