Step 1
Preheat oven to 180°C. Grease the base and side of a 10-cup (2.5L) straight-sided ovenproof casserole or soufflé dish with butter. Combine the breadcrumbs and parmesan in a bowl. Add breadcrumb mixture to the prepared dish. Turn to coat the base and side. Tap to remove any excess breadcrumb mixture.
Step 2
Melt the extra butter in a large saucepan over medium heat. Add the garlic and stir to combine. Add the flour and cook, stirring, for 1 min or until mixture is grainy. Add the milk and whisk until well combined. Cook, stirring constantly, for 2 mins or until the mixture boils and thickens. Remove from heat. Stir in the parmesan, gruyère or Swiss cheese, cheddar, sour cream, thyme, mustard and paprika. Season. Add egg yolks and stir to combine.
Step 3
Use an electric mixer to whisk the egg whites in a clean, dry bowl until firm peaks form. Fold one-third of the egg white into the cheese mixture until just combined. Fold in the remaining egg white. Pour into the prepared dish. Bake for 45 mins or until the soufflé is puffed and golden brown.
Step 4
Meanwhile, to make the celery and hazelnut salad, pick the leaves from the celery sticks and add to a bowl of iced water. Use a vegetable peeler or mandolin to thinly shave the celery sticks lengthways. Add to iced water to curl. Whisk the vinegar, mustard, garlic and oil in a small bowl. Add the lemon juice and season. Drain the celery. Place the celery and lettuce in a medium bowl. Drizzle with dressing and lightly toss to combine. Sprinkle with hazelnut.
Step 5
Serve the soufflé immediately with the celery and hazelnut salad.
Serve with... thyme leaves.