Heat oven to 400.
Bring a large pot of water to a boil with several pinches of salt. Add green beans and cook for about 5 minutes, until they turn bright green. Once cooked, drain and rinse the beans until they are cooled. Set aside.
In a medium skillet, add canola oil to a depth of about one inch and heat. Add the leeks and cook until they are crisp. Remove from oil and put onto a paper towel lined baking sheet. Sprinkle with salt.
Melt butter in a saucepan over medium heat and add mushrooms and thyme. Cook until mushrooms are soft and begin to take on color. Add garlic and cook for a minute or less, being careful not to let garlic burn.
Add flour and cook for about a minute, stirring the whole time. Add stock and half-and-half and stir. Cook for 5 minutes, or until the mixture thickens and no longer tastes of flour, stirring frequently. Season with salt and pepper.
Mix green beans into the mushroom sauce and transfer to a casserole dish. Top with cheese, breadcrumbs and fried leeks.
Bake for 10 minutes or until cheese is melted, breadcrumbs brown and sauce is bubbling.
NOTE: I guessed at prep and cook time as I haven't actually made this yet and the recipe doesn't say.