Trim the root end and the dark green leaves from the leeks and discard. Slice the remaining white and light green parts in half lengthwise and then cut crosswise into thin half-moons. Place in a colander and rinse thoroughly, being sure to remove any sand and grit. Shake off any excess water.
Trim a tiny slice off the root end of each chanterelle. Use a stiff-bristled brush to gently brush any visible dirt off each mushroom. Be sure to check the undersides of the caps, between the "teeth." Speaking of which, a new toothbrush works just fine for this task if you don't have a specialty brush. Chop or tear any large mushrooms into bite-sized pieces.
Preheat oven to 375°F with a rack in the center.
Melt two tablespoons of the butter in a 10-inch skillet over medium heat. Add the leeks, rosemary, thyme, sage, and ¼ teaspoon of the salt. Cook, stirring frequently, until leeks have softened and substantially reduced in volume, about 10 minutes. Remove to a bowl and wipe out the skillet
Melt two more tablespoons of the butter in the same skillet over medium-high heat. Add mushrooms and ¼ teaspoon of the salt. Cook, stirring only occasionally, until they have released and reabsorbed their moisture, reduced substantially in volume, and browned a bit.
In a large bowl, whisk the eggs with a fork, then whisk in the milk, remaining salt, and pepper.
Add the bread cubes and stir gently but thoroughly so that the bread absorbs the milk mixture evenly.
Add the leeks, mushrooms, and gruyere and stir gently until well-distributed.
Use the remaining tablespoon of butter to grease the casserole dish or ramekins.
Spoon the bread mixture into the dish(es), distributing evenly.
Bake for 40 minutes in a large, deep dish like the soufflé dish pictured here, about 35 minutes for a shallower casserole dish, or 30 minutes in individual ramekins, until the top is golden brown and the insides are custardy, with bubbling around the edges.
Let cool for a few minutes before serving.