Honey Balsalmic Glaze:

In a small saucepan, stir together the balsamic vinegar and the honey. Bring to a boil, then reduce heat to low. Simmer for 15-20 minutes, stirring occasionally, until reduced to 3 tablespoons.

While glaze reduces, prepare the beetroot purée.

Beetroot Purée:

Add all ingredients to the bowl of a food processor or high-speed blender and purée until smooth.

Transfer to a bowl and refrigerate until ready to use.

Tart:

Preheat oven to 400° F / 205° C. Line a half-sheet rimmed tray with parchment.

Unroll puff pastry on prepared tray.

With a rubber spatula or small stepped palette knife, spread with the beetroot purée, allowing a 1/2-inch border around the edges.

Layer with the sliced apples, followed by the sliced beets.

Scatter with crumbled feta cheese and chopped walnuts. Season with salt and pepper to taste.

With a pastry brush, brush egg wash along the pastry border, then turn up edges and crimp with a fork.

Brush pastry with more egg wash.

Bake in preheated oven for 25 minutes, or until pastry turns flaky and deeply golden brown.

Drizzle with the honey balsamic glaze and sprinkle with fresh thyme leaves. Slice and serve warm.

Wrap leftover tart in aluminum foil and store in the refrigerator for up to 2 days.