The kielbasa browns first and comes out of the pan, leaving its fat behind to fry the peppers and zucchini. Give the zucchini a minute undisturbed in a wide pan, or it steams soft before it takes any color.

Cut everything before the pan goes on: the kielbasa into coins, the zucchini into half-moons, the peppers into pieces, and the garlic minced. Once the pan is hot, each stage is only a few minutes long, and none of them leaves time to chop the next thing.
Heat half the olive oil in a 12-inch skillet over medium-high. Lay the kielbasa in a single layer and cook about 5 minutes, turning once, until both sides are well browned. It is already fully cooked, so this is for color and for the fat it gives up, not for doneness. Lift it out onto a plate and leave the fat in the pan.
Add the peppers to that fat and cook about 4 minutes, stirring now and then, until they have softened and their edges have started to char. They go in before the zucchini because they take longer to give.
Push the peppers to one side, add the rest of the oil and spread the zucchini out in it. Leave it alone for a minute so the cut faces brown, then stir and cook about 4 minutes more, until it is tender but still holds its shape when pressed. A smaller or crowded pan steams the zucchini soft before it colors, which is why the skillet is a 12-inch one.
Stir in the garlic, and the red pepper flakes if you want some heat, and cook about 30 seconds, until it smells sweet rather than sharp. Return the kielbasa, season with the salt and pepper, and toss until everything is hot through.
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