For roast beef, combine oil, beef, herbs, garlic and pepper in a bowl, cover and marinate overnight, turning occasionally. Preheat oven to 220C, drain beef, place on a wire rack over a baking paper-lined oven tray, season to taste. Roast until cooked rare (20-25 minutes), cover with foil and rest (10-12 minutes), thinly slice and set aside.
For mushroom duxelle, reduce oven to 190C. Combine mushrooms with herbs, garlic and half the oil in a bowl, season to taste. Transfer to a baking paper-lined oven tray, roast until golden (15-20 minutes). Cool slightly, process in a food processor until a fine paste forms (1 minute), then with motor running, gradually add remaining oil. Season to taste and set aside.
For potato crème fraîche, cook potato in boiling salted water until tender (20-25 minutes). Drain, peel and pass through a fine sieve. Fold in remaining ingredients, season to taste and set aside.
Preheat a grill on high heat. Halve rolls, spread bottom halves with duxelle, then layer with beef and Fontina. Spread top halves with potato crème fraîche. Place bottom halves on a baking-paper-lined oven tray and grill until cheese melts (2-3 minutes). Top with rocket, season to taste, sandwich together and serve immediately.