Chop onion and slice zucchini into 1/4-1/2 inch thick slices.
Melt butter and olive oil in a skillet on medium heat.
Add onion and zucchini to pan. Sprinkle with a bit of Italian seasoning.
Cook on medium heat, stirring occasionally, for about 20 minutes, or until zucchini is softened and slightly browned.
Notes
If you don't want the onions and/or the zucchini browned, you can sweat them by keeping the lid on the pan and stirring every 5 min or so until they're cooked the way you want them.