In a bowl, toss cauliflower with olive oil, salt, and pepper. Spread in an even layer on a parchment-lined baking sheet. Roast for 20–25 minutes, stirring halfway through, until edges of the cauliflower start to brown and florets are tender. Remove from oven, let cool for a few minutes, then transfer to a large bowl.
Slice hard-boiled eggs in half, then slice each half into 4 pieces. Roughly chop the bacon slices. Add the eggs and bacon to the bowl, along with cheddar cheese and scallions.
In a separate bowl, whisk dressing ingredients together until smooth. Pour dressing over cauliflower and toss to coat everything well. Garnish with extra sliced scallions before serving.