Soak sweetbreads in bowl of cold water for 1 hour. Drain and place in fresh cold water in saucepan and bring slowly to boil. Drain and place in fresh bowl of water, then drain and remove skin. Slice in 1/2 and set aside.
Meanwhile, heat oven to 400 degrees F. Paint bread with clarified butter and toast for 15 minutes. Remove from oven and lightly paint edges with egg white. Coat edges with most of parsley and keep warm.
Melt butter in saucepan and add flour. Stir to form a roux and cook 1 minute over low heat. Add veal stock, increase heat and whisk until it boils. Season with salt and pepper and add bay leaves. Reduce to simmer and cook 5 minutes.
Heat clarified butter in frying pan over medium heat. Add champignons and onions and fry gently for 2 minutes. Season with salt, pepper and lemon juice. Remove from heat and add to sauce. Add sweetbreads and simmer 5 minutes.
Combine 2 tablespoons cream with egg yolks. Add a little hot sauce to egg/cream mixture and return all to sauce, stirring carefully. Remove bay leaves and add remaining cream to sauce. To serve, fill croustades with sweetbread mixture and sprinkle with parsley and paprika.