Preheat the oven to 400 degrees Fahrenheit. Line a large-rimmed baking sheet with parchment paper.
Peel the carrots and cut them to 2 inch pieces. cut the top and bottom off the onion remove the papery outside layers and then cut into quarters. cut the top off of the garlic bulb. Toss all these vegetables in olive oil then place on the baking sheet sprinkle with salt and peppers. Wrap the garlic in aluminum foil and place on the baking sheet. Put the sheet into the oven, turning every 10-15 minutes, until nicely roasted and softened with some brown spots. Once finished remove from oven.
Set aside the garlic until cool enough to handle. In a Dutch oven on a burner set to medium-low add the roasted vegetables and water or broth. bring to a simmer and cover. Once the garlic is cool enough to handle squeeze the garlic out of the cloves and add to the Dutch oven. Add the coriander and cumin and simmer for 30 minutes. Remove the pan from heat and using an emulsion blender puree the soup completely or let it cool a bit and carefully transfer the soup to a blender, working in batches if necessary. Remember to not fill the blender completely with hot liquids and do not seal the lid for safety. I pull the center off the lid and use a kitchen towel to cover the hole. Pour blended soup into a fine mesh sieve and press the soup back into the pot.
Add the lemon juice, a dollop of yogurt, croutons, and several twists of black pepper. to taste.