Position one of the racks to the bottom third of the oven and preheat to 400°F.

Butter the cocotte and dust with the parmesan, tilting to evening coat the bottom, sides, and rim.

In a heavy-bottomed skillet with lid, heat the oil over medium heat. Add fennel and onion and cook, stirring occasionally, until mixture starts to brown, about 5 minutes. Cover and cook for 5 minutes. Uncover and add 1 tablespoon of water. Cook, stirring, until golden brown and soft. Remove from the pan and let cool.

In a heavy-bottomed saucepan, melt butter over medium heat. Add the flour and stir constantly for about three minutes, until completely smooth. Slowly whisk the milk in. Continue whisking over medium heat for 5-10 minutes until the sauce thickens. Remove from heat and whisk in salt. Add the egg yolks 1 at a time, whisking well after each addition. Use a silicone spatula to scrape the soufflé base into a large bowl. Stir in the cooled fennel and onions, bacon, and chives.

With an electric mixer, beat the egg whites until shiny and stiff, but not dry. Fold in ¼ of the whipped whites into the soufflé base. Fold in remaining whites in two additions, being careful not to deflate. Transfer to the cocottes.

Place the cocottes on a baking sheet and place in the oven. Immediately reduce the oven to 375°F. Bake until puffy and golden and slightly jiggly in the center, about 25-30 minutes. An inserted toothpick should come out fairly dry. Serve immediately.