Preheat the oven to 350 F°.: The moment you preheat, the kitchen begins to smell faintly warm and anticipatory, and your oven reaches the steady environment the flaugnarde needs to set evenly. You want the oven fully warmed so the batter puffs gently and cooks through without overbrowning. A common mistake is putting the dish into a not fully heated oven, which can cause uneven rising and a dense center. Listen for the oven clicking off the preheat cycle, and if you have an oven thermometer, confirm it reads around 350 F° to ensure accuracy.
Lightly spray a baking dish with baking spray and dust with a little flour. Toss in the berries.: The scented oil from the spray and the dusting of flour create a release layer, so the flaugnarde lifts cleanly when sliced. As you toss the mixed fresh berries into the dish, you'll hear a soft patting sound and notice the colorful fruit settling into a casual pattern. This layering helps fruit remain distributed rather than sinking. Avoid packing the berries tightly, because overcrowding can yield soggy pockets rather than beautifully roasted bits. If you skip dusting, the bottom may stick, making serving messy.
Whisk the eggs, agave, salt, and flour together in a large bowl until smooth. Add the milk and vanilla extract. Whisk until smooth then pour into the baking dish.: As you whisk, the batter will take on a satiny texture, no lumps visible, and a faint vanilla aroma will emerge. The batter should be smooth enough to coat the back of a spoon, indicating proper emulsification between the eggs , flour , and milk . Pouring the mixture over the fruit, you will see the berries sink slightly and the batter weave around them. The reason this order matters is to prevent flour lumps and ensure an even bake. A typical error is not fully dissolving the sweetener, which leaves a gritty texture, so whisk until everything seems homogenous.
Bake for 45 - 50 minutes or until lightly browned and a toothpick inserted into the center comes out clean.: During baking the kitchen fills with a warm aroma of cooked berries and vanilla, and the top develops a gentle golden color. You may hear soft crackling as the top sets, and the edges will pull away slightly from the dish. The toothpick test verifies the custard has set without being overbaked. If you bake too long, the texture will dry out, so begin checking around 45 minutes. An often overlooked sign of doneness is a slight jiggle in the center, which is acceptable because it will continue to set as it cools.
When you pull it out of the oven it will wiggle a bit which is completely normal. Cool on a wire rack. The flaugnarde will have puffed up a bit and will deflate while cooling.: As it rests, you will smell intensified berry sweetness and notice steam gently rising. The puffing and deflating are part of the custard setting process; it indicates air pockets collapsing and the structure stabilizing. Cooling on a wire rack prevents sogginess from trapped steam under the dish. Resist the urge to cut into it while piping hot, because the interior needs time to finish setting. A common pitfall is serving too soon and getting a runny interior, so be patient for a firmer texture.
When cool, dust with powdered sugar and serve.: The final dusting of powdered sugar adds a refined visual contrast and a whisper of sweetness on top. As you slice, the surface yields a gentle crack and the fragrant berry aroma becomes more pronounced. Serve warm or at room temperature to catch the best texture, and observe the interplay of custard and fruit in each forkful. Avoid pre dusting too early, as moisture will dissolve the sugar and dull the presentation.