Preheat oven to 450 degrees F. Bring olive oil, balsamic, and 7 1/2 cups of water to a boil in a large pot. Add salt and cauliflower. Bring back to a boil. Reduce to a gentle simmer and place a plate on top of cauliflower to keep it submerged. Simmer for 12 minutes until softened.
Using 2 slotted spoons or mesh sieve, transfer cauliflower to a small rimmed baking sheet and let cool.
In a medium bowl, combine spinach, eggs, green onion, garlic cheddar, cream cheese, panko and Parmesan. Place in a piping bag with a 3/4 inch tip.
Position cooled cauliflower stem side up on a rimmed baking sheet. Pipe filling between stalks of florets. Flip cauliflower stem side down, then lay strips of bacon, just slightly overlapping the strips over the cauliflower, tucking ends of strips into the bottom of the cauliflower. Roast, rotating 1/2 way through until bacon is cooked, about 30 minutes.