Heat olive oil in a large pot. Add onion; cook and stir until browned, about 10 minutes. Add 1 cup water and carrots. Bring to a boil; cover and cook until carrots are soft, about 10 minutes.
Combine remaining 4 cups water, cauliflower florets, chicken bouillon, and garlic in a blender; blend until smooth. Pour over the onion and carrots in the pot.
Stir baby clams and mushrooms into the pot. Simmer until hot, about 10 minutes. Season with salt and pepper.