In a large saucepan, heat oil, then add garlic and leek. Cook until leek softens, stirring often. Add mushrooms, cook until mushrooms soften, stirring often.
Remove pan from heat, sprinkle flour evenly over and stir to distribute through mixture. Return to heat for 1 minute. Gradually add milk and stock while stirring, then cook until liquid parts of mixture thicken.
Add peas, chives and mustard. Cook another 2 minutes or until peas thaw.
Line the bottom of 4-5 ramekins (1-1.5 cup capacity is ideal) with a sheet of mountain bread. The sheet will pleat as you press it into the dish, allowing the sides to come up and form a cup. Distribute the leek, mushroom and pea mixture evenly among dishes, up to the top.
Top each dish with additional mountain bread, torn into small pieces to form the "lid". Spray mountain bread lightly with oil and place ramekins on a baking tray.
Bake for 10 minutes, uncovered, until mountain bread is brown and crisp. Serve.