Preheat the oven to 350°F. Heat the oil in a large, deep ovenproof skillet over medium heat. Add the leeks and sauté for about 6 minutes, until softened. Stir in mushrooms and garlic, season with salt and pepper, and sauté until the mushrooms have released their liquid, and it has evaporated, and the mushrooms have started to brown, 8 to 10 minutes. Turn the mushrooms and leeks into a bowl.
In the same skillet, heat the butter over medium heat, then add the flour, and whisk until it becomes a golden brown paste, about 2 minutes. Slowly add the broth, whisking frequently, season with salt and pepper, and bring to a simmer allow it to thicken. Add the carrots, broccoli, and salmon and simmer for 3 minutes, stirring occasionally then add the peas, dill and lemon zest, as well as the cooked mushrooms and any accumulated juices, to the skillet and stir to blend. You can proceed with the mixture in the pan, or turn it into a baking dish.
Separate the refrigerated biscuits and evenly distribute them over the top of the salmon pot pie mixture. Brush the top of each biscuit with the beaten egg, and sprinkle lightly with salt. Bake for about 25 to 30 minutes, until the biscuits are golden brown and the filling is bubbly. Scoop the stew and a biscuit into each individual bowl.