1.Preheat oven to 220°C (200°C fan-forced).
2.Make béarnaise sauce. Bring vinegar, peppercorns and onion to the boil in small saucepan. Reduce heat and simmer uncovered, about 5 minutes or until liquid has reduced by half. Strain over medium heatproof bowl, discard peppercorns and onion. Whisk yolks into liquid in bowl until combined
3.Set bowl over medium saucepan of simmering water, gradually whisk in melted butter in thin, steady stream until mixture thickens slightly. Remove from heat, stir in tarragon. Cover to keep warm.
4.Cut potatoes into 1cm slices. Heat butter and half of the oil in large non-stick frying pan, cook potato, uncovered, turning occasionally, until browned. Reduce heat, cook potato, covered, turning occasionally, about 15 minutes or until tender. Stir in chives.
5.Meanwhile, heat remaining oil in medium flameproof casserole dish, cook beef, uncovered, until browned all over. Place dish in oven, roast beef, uncovered, 10 minutes.
6.Add tomatoes to dish, roast, uncovered, about 10 minutes or until beef is cooked as desired and tomatoes are soft. Remove beef and tomatoes from dish and cover to keep warm.
7.Make mushroom sauce, using dish with beef juices. Place same flameproof casserole dish with beef juices over medium heat, add onion and mushrooms; cook, stirring, until onion softens.
8.Add stock, wine and cream and bring to the boil. Reduce heat and simmer, uncovered. Stir for about 10 minutes or until sauce thickens slightly and mushrooms are tender. Cover to keep warm.
9.Boil, steam or microwave carrots, broccolini and squash, separately, until just tender, drain.
10.Serve beef with potato and vegetables on large serving platter accompanied with both sauces