Trim the stalks from the beetroot bulbs, reserving the leaves, then slice into 1cm pieces. Place in a baking tin, add 2 tablespoons of olive oil and fill the tin to a depth of about 5cm with water.
Cover the dish with tin foil, then bake for 1 hour to 1 hour 15 minutes, or until a knife slips easily into the flesh and the skins slide off. Remove the beets from the tin and leave to cool.
Peel and chop the onions into wedges, peel and finely chop the garlic, then drain and rinse the capers. Pick and chop all the herbs.
On a baking tray, toss the onion wedges in 2 tablespoons of the olive oil and season well. Add these to the oven too, and roast for about 30 minutes, or until soft. Remove and set aside to cool.
When the beetroot is cool enough to handle, rub off the skins, then cut into wedges (wear plastic gloves for this, or your hands will be pink for days). Set aside.
Blanch the beetroot stalks and leaves in a pan of boiling salted water for about 2 minutes, then drain well.
Heat the remaining tablespoon of oil in a pan over a high heat, add the beetroot stalks and garlic and fry for a few minutes, until the garlic is golden. Then turn down the heat to medium, pour in the wine and cook for 10 minutes. Add the beetroot leaves, season, and cook until wilted.
To make the vinaigrette, whisk the vinegar into the mustard, then stir in the olive oil and season to taste. Set aside.
Now put the salad together. In a large serving bowl, gently toss the roasted beetroot and red onions with the stalk mixture, chopped herbs, capers and vinaigrette, then mix through the watercress and serve.