Preheat oven to 350F. Toast almonds on a baking sheet in centre of oven, 2 to 3 min. In a small bowl, whisk vinegar with honey and salt. Slowly whisk in oil.
Slice a small piece off top and bottom of each orange. Slice off and discard remaining peel, including white pith. Slice oranges into thick rounds. Cut onion into rounds, then separate into rings. Arrange mâche, orange rounds and onion rings on a large platter. Sprinkle with almonds and drizzle with dressing.