Preheat the oven to 375°F (190°C). Place the pre-made pastry crust into a 9-inch tart pan and set it aside.

In a skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for 3-4 minutes until soft and translucent.

Add the minced garlic to the skillet and cook for an additional minute until fragrant.

Stir in the fresh spinach and cook until wilted. If using frozen spinach, heat until warmed through. Season with salt and pepper, then remove from heat and allow it to cool slightly.

In a mixing bowl, whisk the eggs and heavy cream together until smooth. Season with a little salt and pepper.

Add the grated Irish cheddar cheese and the sautéed spinach mixture to the egg and cream mixture. Stir gently to combine.

Pour the filling into the prepared pastry crust and spread it out evenly. Bake for 30-35 minutes, or until the tart is set and golden brown on top.

Allow the tart to cool for a few minutes before slicing. Serve warm or at room temperature. Garnish with extra grated cheese or fresh herbs if desired.