Pierce the potatoes several times, and place on a microwave safe plate. Cook potatoes on high in microwave until tender when pierced with a fork, about 5 minutes (cooking time will vary depending on the size of the potatoes and the power of the microwave). Coarsely chop the potatoes into approx 1-inch chunks.
Combine potatoes, red pepper, broccoli, scallions, and broth in large pot over medium-high heat. Bring to a boil. Lower heat to simmer, and cook 10 minutes, stirring occasionally and mashing potatoes with the back of a wooden spoon.
In a small bowl, stir together milk, 1/4 cup sour cream, cornstarch, salt and pepper. Stir milk mixture into soup. Taste for salt and pepper. Also, if the soup is too thick add additional broth or some water to thin it out.
Spoon soup into bowls, dollop each with sour cream and sprinkle with cheddar cheese, if desired.
Cook's notes: I love baked potatoes with the skins on. And, the skins have so much nutrition in them. I think this soup benefits from having the additional body and texture that the skins bring. But, if you want a smooth silky soup, peel the potatoes before cooking them in the microwave.