In a large saucepan or wok, melt 3 tablespoons of the butter over medium-high heat. Add the olive oil and the onion and cook for 3 to 4 minutes, until onion begins to soften. Turn heat to high and add the mushrooms. Cook, stirring, for 3 to 4 minutes, until mushrooms begin to brown. Add the remaining tablespoon of butter and the zucchini and cook, tossing the vegetables every minute or so until the zucchini is cooked through, about 6 minutes. Add the corn and toss until the corn is heated through. Remove from the heat, add the dill and salt and pepper to taste, and toss. Serve immediately.