Preheat oven to 425. Toss the squash onto a sturdy baking sheet, drizzle with olive oil and swoosh the veg around to get a good coating. Season very well with salt, pepper, chile flakes, and 1-2 sprigs rosemary. Roast for 30 minutes or until tender. Remove from oven and let cool a few minutes. If you want to top the dip with some whole pieces of butternut set a handful aside now.
In a large food processor dump the rinsed beans, garlic, squash and salt. Zest and juice the lemon into the processor. Coarsely chop another sprigs worth of rosemary and add that as well.
Blend everything 'til finely ground. Give it a stir and put the lid back on. With the motor running, drizzle the olive oil steadily in, then add ice water until you've got a good hummus-like consistency.
Taste and season. Add chile flakes, pepper, more salt and rosemary as desired.
Plate in a shallow bowl with an extra sprinkle of flaky sea salt, drizzle of olive oil, and the remaining butternut bits.