Heat a dutch oven over high heat. When the pot is hot, spray it with cooking spray. Add the broccoli, onion, and garlic. Cover, and sauté the vegetables until they are almost tender, about 5 minutes. Season lightly with salt. Add the chicken broth, bring the mixture to a boil, and cook uncovered until thw liquid has reduced by half, about 8 minutes

Meanwhile, whisk the milk and cornstarch together in a medium bowl until theoughly combined.

Whisk the cornstarch mixture into the broccoli mixture. Bring the soup to a boil and cook, stirring often, until it has thickened, about 2 minutes

Pour half the contents of the pot into a blender, and puree until smooth. Pour the pureed soup back into the pot. Adjust the consistency with a little more chicken broth, if necessary.

Remove the pot from the heat. Stir the cheese and yogurt into the soup. Season with salt and pepper to taste, and serve.