Kale and Butternut Squash Gratin

bunch Tuscan kale, stemmed and leaves chopped

(2 oz./60 g) unsalted butter

shallot, finely chopped

(1 1/2 oz./45 g) all-purpose flour

(8 fl. oz./250 ml) chicken stock

(8 fl. oz./250 ml) heavy cream

(10 oz./315 g) grated Gruyère cheese

chopped fresh sage

Kosher salt and freshly ground pepper

butternut squash, peeled, seeded and cut crosswise into half-moons 1/2 inch (12 mm) thick

(1 oz./30 g) panko (Japanese bread crumbs)

(2 oz./60 g) grated Parmesan cheese