(I find it best to slice up the entire baguette (long French loaf) because if you use only what you need, the rest of the loaf goes stale. Bake the bread, cool the slices you don't use, and keep in a plastic bag to use again -- can be heated up again).
Heat oven to 400 degrees F.
Slice the baguette at a slight diagonal slant.
Pour some fruity virgin olive oil in a bowl, dip each slice just lightly into the oil (one side only). It does not have to be covered in oil -- just "anointed": it will have patches the oil didn't reach.
Pack on any cookie tin, and when oven has reached its heat, bake the slices until the edges start to go golden, and the slices are hard in the centre. (Do feel before you remove them!).
Season with salt and black pepper, and trust me, a pure flaky sea salt is better tasting than ordinary kitchen salt.
Use as many baked bread slices as you need -- at least 3 per person. As you add the topping, put them back on the baking tin, or transfer to another, smaller dish -- depending on how many you are making.
Spread the hot, hard baguette slices thinly with pesto, and top each with a teaspoon or more of the tomato-onion mix.
Cool the slices you did not use, and keep in an airtight container.