Preheat the oven to 170°C / 340°F and line a baking sheet.

Peel the butternut squash, red beetroot, and parsnip. Cut them into 1 cm / 0.4 in thick rounds of matching diameter.

Arrange the vegetable rounds on the baking sheet. Season with 2 tbsp / 30 ml olive oil, salt, black pepper, 1 crushed garlic clove, and sage.

Roast the vegetables for 30 minutes at 170°C / 340°F. Let them cool until easy to handle.

Soak the bread crumb in 100 ml / 3.4 fl oz milk. Drain it after it has absorbed the milk.

Add the soaked bread crumb, walnuts, Parmesan cheese, 1 garlic clove, and 3 tbsp / 45 ml olive oil to the food processor. Blend to a coarse, creamy pesto.

Stir lemon juice into the walnut pesto. Adjust the seasoning with salt and black pepper.

Bring salted water to a boil in the small saucepan. Blanch the cabbage leaves for 30 seconds, then drain them.

Cut 8 puff pastry rounds large enough to enclose four individual stacks. Place 4 rounds on the lined baking sheet.

Layer roasted vegetable rounds onto each pastry base. Add a cabbage leaf generously spread with walnut pesto between the vegetable layers.

Cover each stack with a remaining pastry round. Seal the edges firmly with a fork.

Make a small steam hole in the top of each pithivier. Brush each pastry with the egg yolk mixed with the remaining 25 ml / 0.8 fl oz milk.

Chill the pithiviers for 30 minutes if decorating them. Score the chilled pastry lightly with a paring knife if desired.

Bake the pithiviers at 180°C / 355°F for 30 minutes. Remove them when the pastry is deeply golden and crisp.

Rest the pithiviers for a few minutes. Cut each one in half with a bread knife and serve warm.