Generously smear one side on six slices of the brioche bread with the blueberry jam (about 1½ tablespoons each). Sandwich with another piece of bread.
In a large bowl, whisk the eggs with the heavy cream, sugar, lemon zest, and salt.
In a large nonstick skillet, melt 1 tablespoon of butter. Working in batches, dunk the brioche sandwiches in the custard until saturated on both sides, and then fry the French toast over moderately-high heat, flipping once, until golden brown, about 2 minutes per side. Transfer to a large rimmed baking sheet. Repeat with the remaining brioche sandwiches and custard, adding more butter to the pan as needed.
Transfer the rimmed baking sheet with the French toast to the oven and bake for about 5 minutes, or until the French toast is cooked through. Transfer to plates.
In a microwave-safe bowl, heat your desired amount of blueberry jam until smooth and runny, about 1 minute. Dust your French toast with confectioners’ sugar, if desired, then drizzle with jam.
Top each slice with a crowning dollop of fresh ricotta and garnish with fresh blueberries, toasted chopped pecans, and more fresh lemon zest, if desired. Sprinkle with a little flaky sea salt and serve right away with more jam on the side.