• Preheat the oven to 425°F.

Caramelize the Onions

• Truely caramelizing onions takes 40-50 minutes. You may want to make a big batch ahead of time. If you do, measure about 2/3 of a cup of caramelized onions. Slice the large onion through the poles. Use a 1 quart sauce pan to melt a tablespoon of butter on medium-high heat. Add the onion and 1/2 tsp of salt to the pan. It doesn't require much stirring early on, but as the water is released and evaporates, you'll need to pay attention with regular stirring and scraping. A brown fond devlopes on the bottom and side of the pan. Scrape that up and incorporate into the onions. If things get away from you, take off heat, add just a tablspoon of water, stir, and let sit for a minute. You should be able to scrape the fond into the onions and then continue cooking. After 40-50 minutes, you should have about 2/3 of a cup of caramelized onions.

Prepare the tomatoes

• You have the choice of slicing and chopping, or just chopping. Involving slicing provides a little bit nicer presentation for a little bit more work. Aim for a little over half of the tomatoes to be prepared for roasting and a little under half to be prepared for weeping. If you are slicing and chopping, the slices will be roasted and the chopped will be weeped. If you slice, also score the edges of the slices in several places such that there isn't a single circular ribbon of skin.

Roast two thirds of the tomatoes

• Whether sliced or chopped, spread the tomatoes evenly around a baking sheet. Parchment paper is optional. It can make it easier to transfer the tomatoes after roasting completes. Sprinkle ½ tsp of salt over the tomatoes and transfer to the pre-heated oven. Roast for about 30 minutes. You are looking to evaporate some water and get a little bit of browning color.

Weep one third of the tomatoes

• Put the remaining chopped tomatoes in a large mixing bowl. Add ½ teaspoon of salt and ½ teaspoon of sugar and stir to mix. The salt and sugar will both draw out water from the tomatoes via osmosis. Transfer the tomatoes to a colander or to the basket of a salad spinner. Save the mixing bowl for the next step. Allow to drain for 30 minutes. If you are using a salad spinner, spin the tomatoes a few times after 30 minutes,

Assemble and bake the galette

• Once you have the onion caramelized and half of the tomatoes roasted and the other half weeped, you are ready to bring the galette together.
• If you chose to slice the tomatoes that were roasted, reserve them for a later step. Mix all of the chopped tomatoes with the onions, mayonnaise, rosemary and ⅕ of the grated cheese.
• Roll the dough out to 14" in diameter and transfer to a rimmed baking sheet. Parchment paper is optional, but it can make things easier to move after baking.
• Spoon the tomato mixture onto the center of the dough circle and spread around leaving a two inch rim at the edge. If you have reserved the sliced roasted tomatoes, spread them evenly over the surface of the mixture. Sprinkle the remaining ⅖ of the cheese over the top.
• Create a rustic rim by folding the remaining dough over the edge. Fold the first flap, and then iteratively fold overlapping flaps, until the rim has been built.
• Beat an egg, and use a pastry brush to apply a thin egg wash to all exposed dough.
• Transfer to the 425 F oven and bake until the crust is golden brown. Expect 35-45 minutes.
• Allow the galette to cool for 10 minutes before attempting to cut and serve.