Preheat your oven to 400°F (200°C) and prepare a baking sheet by lining it with parchment paper.

In a large skillet, heat olive oil and butter over medium heat. Add your thinly sliced onion, sauté gently for 15-20 minutes.

While the onions caramelize, cook the chopped bacon in another skillet until crispy, about 6-8 minutes.

On a lightly floured surface, roll out the puff pastry until it's large enough for the baking sheet.

In a mixing bowl, whisk together the pumpkin puree and ricotta cheese until smooth. Season with salt and pepper.

Spread the pumpkin-ricotta filling evenly onto the puff pastry, leaving a 1-inch border. Scatter onions and bacon on top.

Brush sage leaves with melted butter and arrange over the tart.

Fold the edges of the puff pastry inward to create a rustic border and brush this border with beaten egg.

Bake the assembled tart in the preheated oven for 20-25 minutes.

Let the tart cool for about 10 minutes on the baking sheet before slicing and serving warm.