Cook the onions and tomatoes in the olive oil on medium heat for 10 minutes. Season with thyme, salt and pepper. Spread the vegetables in the pie shell and bake at 400° for 20 minutes.
Remove the sardines from their cans and break them into bits (it's fine to leave the bones in the sardines). Remove the tart from the oven, spread the sardine bits evenly across the surface and bake for five minutes longer.
