Melt the butter in a large pan over medium heat.
Add onion and cook until soft, about 8 minutes.
Add zucchini and yellow squash to the pan and cook, stirring occasionally, until it just begins to soften. Anywhere between 7 and 10 minutes.
Add the fresh corn kernels and thyme. Stir to combine and cook for another 1-2 minutes. Turn off the heat and stir the chopped tomatoes into the zucchini, onion, corn mixture. Let sit for 5 minutes in the pan - just long enough to heat the tomatoes and warm them through.