Heat the water and vinegar or lemon juice in a large saucepan. Add the onion, cover and cook, stirring occasionally, until tender and lightly browned (about 15-20 minutes). If the onion sticks to the pan, add a few teaspoons more water to help loosen it.

Meanwhile, cook the macaroni in boiling water until al dente. Drain and stir into the cooked onions. Mix well.

Process the remaining ingredients for several minutes until completely smooth. Stir the blended mixture into the macaroni and onions and spoon into a lightly oiled or non-stick 3-quart casserole dish. Bake uncovered for 25-35 minutes. Serve immediately.