Peel the carrots and beetroot and cut them into 6 to 8 equal strips. Wash the sweet potatoes and cut them into strips of 1x1 cm.
Sauté the sweet potato slices until golden brown. Add the carrots and beetroot and sauté them until tender. Season them immediately with some salt and chopped rosemary. Leave them to cool a little.
For the honey-mustard dressing, mix the mustard, honey, lemon juice and olive oil and whisk together in a bowl. Season with salt and pepper.
Wash the watercress and set aside the tops. Drain the cheese and pat dry.
Arrange the sweet potatoes, beetroot and carrots on a platter and crumble the feta cheese on top.
Add the watercress tops and drizzle the honey-mustard dressing over everything.