Preheat oven to 450°. 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 center, about 12 minutes.
Using 2 slotted spoons or a mesh spider, transfer cauliflower to a small rimmed baking sheet. Let cool.
Meanwhile, combine spinach, eggs, green onions, garlic, cheddar, cream cheese, panko, and Parmesan and place in a piping bag with a ¾-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 strips, over cauliflower, tucking ends of strips into bottom of cauliflower
Roast, rotating sheet halfway through, until golden all over, about 30 minutes.