Soak the onions if using in a bowl of ice water to take out the bite, 5 to 10 minutes.
Toast the cumin and coriander seeds in a dry skillet over low heat until fragrant, about 2 minutes. Smash slightly on a cutting board with the bottom of your cooled skillet or with the back of your knife.
Toast the bread. Spread the toast with a thin schmear of butter or a drizzle of oil.
Drain the onions well. Toss together the cumin and coriander seeds, onions, tomatoes, oil and salt in a bowl. Stir in the herbs. Spoon over the toast and drizzle very lightly with a bit of honey.