Step 1: Add the Broccoli to the Crock Pot - Pour the frozen broccoli florets straight into the crock pot. There is no need to thaw them first, which is one of the reasons I like this recipe so much on a busy day. Spread them out in a fairly even layer so the smaller and larger pieces are mixed together. As the soup cooks, the broccoli will release a little moisture and soften until you can easily break a floret with a fork.
Step 2: Add the Celery, Onion, and Seasonings - Add the finely chopped celery and diced yellow onion over the broccoli, then sprinkle in the garlic powder, nutmeg, black pepper, and chicken bouillon powder. I like to scatter the seasonings over the whole surface instead of dropping them in one spot because it makes the first stir easier. The nutmeg may seem like a tiny detail, but that little bit of warmth works especially well with creamy cheese sauces and soups.
Step 3: Add the Flour and Creamy Liquids - Sprinkle the flour evenly over the vegetables, then pour in the evaporated milk and chicken broth. Give everything a thorough stir, making sure you reach down to the bottom and pull the flour into the liquid. You do not need the mixture to look completely smooth at this stage. The goal is simply to avoid leaving a dry pocket of flour sitting on top. As the soup heats and cooks, the flour will help the liquid develop more body.
Step 4: Slow Cook Until the Broccoli Is Tender - Cover the crock pot and cook the soup on low for 3 to 4 hours. Try not to lift the lid repeatedly, since every peek lets heat escape and can stretch out the cooking time. Around the 3 hour mark, check a larger piece of broccoli with a fork. You want it tender enough to break apart easily but not cooked so far that every floret completely disappears into the broth. If it still feels firm in the center, cover the crock pot again and give it more time.
Step 5: Stir in the Parmesan and Cheddar - Once the broccoli is fork tender, add the grated Parmesan and the 2 cups of freshly shredded cheddar. Stir slowly until the cheese begins melting into the hot soup. Keep the crock pot on low just long enough for the cheese to melt completely, stirring once or twice if needed. Avoid cranking the heat higher at this point. Gentle heat helps the cheese melt smoothly and keeps the creamy base from becoming grainy.
Step 6: Finish, Taste, and Serve - When the cheese has melted, give the soup one final stir and taste a spoonful. The chicken bouillon and cheeses both bring salt, so you may find that it needs nothing else. Ladle the soup into bowls and top each serving with a little extra freshly shredded cheddar. I usually put the cheese right on while the soup is steaming so it softens into those melty little strands on top. Serve it warm and enjoy the quiet moment when everyone starts eating and the room gets noticeably calmer.