Heat the oven to 450 degrees F.
Combine the squash, 1/4 cup olive oil, chile flakes, and 2 tsp of salt in a bowl and toss well. Transfer the mixture to a parchment-lined baking sheet and cook, stirring every few minutes, until tender and slightly colored (about 15 minutes).
Remove from oven.
Heat another 1/4 cup olive oil over medium-high heat, add the onions and remaining teaspoon of salt and cook, stirring frequently, until the onions are well softened and darkening (about 15 minutes).
Add the vinegar and syrup, stir and reduce until syrupy and broken down (again, about 15 minutes or until the mixture is jammy).
Combine the squash and onions in a bowl and smash with a fork until combined.
Taste for seasoning.
Place a spoonful of the squash and onion jam on crostini, then top with cranberry feta.
Garnish with mint.