Preheat oven to 220c. Bring 8 cups water, oil, lemon juice, and 2 tablespoons salt to a boil in a large pot.
Add cauliflower and bring back to a boil. Reduce to a gentle simmer and place a plate on top of the cauliflower to keep it submerged. Simmer until a knife easily inserts into the centre, about 12 minutes.
Using 2 slotted spoons or a mesh spider, transfer cauliflower to a small rimmed baking tray. Let cool.
Meanwhile, combine spinach, eggs, green onions, garlic, cheddar, cream cheese, panko, and Parmesan and place in a piping bag with a 2cm tip.
Position cooled cauliflower stem side up on baking tray. Pipe filling between stalks of florets. Flip cauliflower stem side down, then lay strips of bacon, just slightly overlapping strips, over cauliflower, tucking ends of strips into bottom of cauliflower.
Roast, rotating sheet halfway through, until golden all over, about 30 minutes.
Enjoy!