1-Getting started with this caramelized onion tart recipe is straightforward and fun. Begin by setting up your workspace, known as mise en place, to make the process smooth. Preheat your oven to 400 degrees F and gather your ingredients for the pastry and filling.

2-Prepare the Pastry: Mix 1 1/4 cups all-purpose flour, 1 teaspoon sugar, and 1/2 teaspoon sea salt in a bowl. Fold in 8 tablespoons unsalted butter cut into 1/2-inch cubes until it forms a clumpy dough with some larger pieces. Gradually add 1/3 cup ice water until the dough holds together, then shape it into a disc, wrap it, and chill for at least 1 hour or up to 3 days. Chilling helps create a flaky base that contrasts nicely with the soft onions.

3-Make the Filling: Remove the cores from 4 medium yellow onions and slice them 1/4 inch thick. Cook the onions in 2 tablespoons unsalted butter over medium heat in batches for 10 minutes until they start to color and sizzle. Reduce the heat, add salt and black pepper to taste, plus 2 bay leaves, and cook for 40 minutes, stirring often and adding water if needed, until the onions turn sweet and golden. Let them cool slightly for easier handling. Remember, caramelizing onions takes time but rewards you with that deep flavor everyone loves in a caramelized onion tart with Gruyere cheese. For more tips on this step, check out a guide on caramelizing onions properly.

4-Assemble and Bake the Tart: Roll the chilled dough into a 12 by 15-inch rectangle on parchment paper and place it on a baking sheet. Spread 1 tablespoon ground Dijon mustard on the dough, leaving a 2-inch border. Scatter 1/4 cup grated Gruyere and 1/4 cup grated Parmesan over the mustard, then add the caramelized onions (remove bay leaves if you prefer). Brush the border with the lightly beaten egg, fold up the sides, and brush the edges again. Finish by brushing the top with the remaining egg, sealing the corners, adding black pepper, and scattering the leaves from the 4 thyme sprigs along with the remaining 1/4 cup grated Gruyere and 1/4 cup grated Parmesan.

5-Bake for 20 to 25 minutes until deep golden brown and puffed, then cool for 30 minutes before slicing. Garnish with extra thyme sprigs for a fresh touch. If you’re looking for complementary recipes, try our creamy chicken mushroom pie for a hearty pairing that adds variety to your meals.

6-Additional Tips for Baking: The preparation takes about 20 minutes, with cooking around 1 hour, plus chilling time. For even results, rotate the baking sheet halfway through. This step-by-step guide ensures your caramelized onion tart with fresh herbs turns out just right every time.