In a pan on medium low, heat olive oil and cook onion, stirring often, until very golden brown (I will not lie to you like so many recipes, this takes a long time so be patient). Stir in thyme, black pepper, and marmite. Remove from heat and reserve.
In a separate pan, heat olive oil, and sauce shallot until translucent. Stir in celery, carrot, potato, crab meat, Old Bay and lemon zest. Remove from heat and reserve.
Make a roux by stirring together butter and flour in a small saucepan over medium low heat. Stir until it is a light golden color, remove from heat, and reserve.
Heat oven to 375. Add 1 1/2 cups vegetable broth and half of the roux to each pan, and simmer each until each mixture is very thick.
Divide the onion mixture between two ramekins, and the crab mixture between two more. To the onion ramekins, top with croutons and a sprinkle of gruyere.
Roll out the puff pastry and cut circles the size of the ramekins. Top each, brush with egg white, cut a couple vents for each, and bake for 15-20 minutes, until golden brown.