Peel the potatoes, cut the large ones if necessary, drizzle with oil and toss. Roast in a moderately hot oven at 200 °C /gas mark 6 for about 25 minutes.
Combine olive oil with lemon juice and Vegeta and mix to blend. Brush on the salmon steaks.
Remove the potatoes from the oven, and add the salmon, skin-side down. Continue baking for another 10-15 minutes. Meanwhile, make the sauce.
Heat a skillet and add the butter. Add the flour and sugar and stir until the butter melts and blends with the flour. Add the milk and stirring all the time, bring the sauce to boil. Season with salt, add the lemon juice, mustard and horseradish, and continue to simmer for a couple of minutes.
Turn off the heat before adding chopped dill and cream. Stir to combine and serve the sauce with baked salmon steaks and potatoes.
Before popping the salmon into the oven, put a cup of water in the oven. The evaporating moisture will prevent the salmon from drying out.