Preheat the oven: Place an oven rack in the centre position and preheat to 400°F (200°C). Line a large baking sheet with parchment paper.

Sauté the vegetables: In a 12-inch (30 cm) oven-safe skillet, heat 1 tablespoon of the grapeseed oil over medium-high heat. Add the onion and sauté, stirring frequently, until golden brown around the edges, 3 to 5 minutes. Add the carrots and mushrooms and sauté, stirring occasionally, until the mushrooms are lightly browned, 3 to 4 minutes. Add the garlic and sauté, stirring constantly, until fragrant, 1 to 2 minutes.

Simmer the filling: To the skillet, add the remaining 1 tablespoon grapeseed oil and the flour, and toss to coat. Add the vegetable stock, stirring to incorporate the flour until no clumps remain. Add the potatoes, thyme, salt, and pepper. Bring to a boil, then reduce the heat to medium and simmer, stirring frequently, until the potatoes are fork-tender, about 15 minutes. Stir in the navy beans, peas, and chives. Remove from the heat.

Meanwhile, prep the puff pastry: Unroll a pastry sheet onto a lightly floured work surface. Using a rolling pin, gently smooth out any creases. Cut the pastry into nine 3-inch (8 cm) squares. Repeat with the other pastry sheet. Transfer the pastry squares to the prepared baking sheet and freeze for 10 minutes.

Assemble and bake: Layer the chilled pastry squares, slightly overlapping, over the filling in the skillet (you may have some squares left over 3 ). Transfer the skillet to the oven and bake until the puff pastry is golden brown about 30 minutes.