Brush the beef with oil and rub in the thyme, salt and pepper. Cook on a medium barbecue for 3—4 minutes on each side. Rest in foil for 5 minutes, then slice thinly.
Meanwhile, to make the dressing, whisk the crème fraîche, mustard, vinegar and chives together, then gradually whisk in the measured water and some salt and pepper until evenly blended.
Toast the bread slices on the barbecue for 1 minute on each side until browned. Rub with the garlic and drizzle with oil. Top each with some salad leaves, capers and beef slices and pour the dressing over.