Preheat the oven to 220C, fan-forced. Cut the unpeeled pumpkin into 2cm chunks (see cooking tip). Finely chop the rosemary, discarding the stems. Crush or finely chop the garlic. Halve the leek lengthwise and thinly slice the pale stem, discarding any dark green stem (see cooking tip). Thinly slice the silverbeet, keeping leaves and stems separate.
Step 2 — Roast pumpkin
Line an oven tray with baking paper. Put the pumpkin on the lined tray. Drizzle with 2 tsp olive oil, scatter with the rosemary and half the garlic, season with salt and pepper and toss to coat. Roast for 20 mins or until golden.
Step 3 — Prep stock and lemon
While the pumpkin is roasting, crumble 1 stock cube* into a heatproof jug, add 250ml (1 cup) boiling water and stir to dissolve. Finely grate the zest of half the lemon*, then juice the half. Bring a large saucepan of salted water to the boil for the ravioli.
Step 4 — Start sauce
Put the walnuts in a cold medium deep frypan over medium heat. Toast, tossing, for 3-4 mins until golden. Remove from the pan and cool slightly, then coarsely chop. Heat 2 tsp olive oil in the pan over medium-high heat. Cook the leek, silverbeet stems, remaining garlic and ½ tsp fennel seeds*, stirring occasionally, for 6-7 mins until the leek is softened.
Step 5 — Finish sauce
Add the silverbeet leaves and stock to the leek mixture. Cook, stirring, for 2-3 mins until the silverbeet is wilted and the stock is reduced by half. Add half the roasted pumpkin and lightly crush with a fork to thicken the sauce. Stir 1 tsp lemon juice* and 2 tsp extra virgin olive oil into the sauce. Taste, then season with salt and pepper.
Step 6 — Cook ravioli and serve
Meanwhile, cook the ravioli in the pan of boiling water for 3 mins or until al dente. Drain. Add the ravioli to the pumpkin sauce and gently toss to coat. Divide the ravioli and remaining roasted pumpkin among bowls. Crumble over the feta and scatter with the walnuts and lemon zest to serve.