Tomato concassé is just a faster (and fancier) way of saying “peeled, de-seeded, and diced”. The tomatoes are blanched in boiling water for up to 30 seconds, quickly moved to an ice bath, then peeled. The same blanching technique can be used for other fruits like peaches or even pearl onions.
Tomatoes have a strong skin and they’re full of juice and seeds, things that don’t necessary go well with things like sauces (Chicken Provençal for example) or a delicate omelette, so tomato concassé is sometimes a better alternative to plain ol’ diced tomatoes.
Heat a large sauce pot of water to a boil. (You want enough water to keep the water boiling even after adding all your tomatoes.)
If the tomatoes have stems, remove them. Score an X into the skin on the bottom of each tomato. You don’t need to cut into the flesh, just the skin.
While you’re waiting for the water to boil, get a large bowl of ice water ready. You’ll use this to stop the tomatoes from cooking and getting mushy.
Boil the tomatoes until the edges of the skin around the X start to peel up. If your tomato is very ripe, this may only take 10 seconds. If you have a cool, moderately ripe tomato, it will take about 20 seconds. If you have a cold, unripe tomato, it will take about 30 seconds.
Remove the tomatoes from the boiling water and immediately submerge them into the ice water.
Don’t run off though! Remove the tomatoes as soon as they’ve cooled down enough to handle. If you leave them in there too long, they’ll absorb a lot of water and dilute the flavor.
Use the edge of a pairing knife to peel the skin off of the tomatoes. It will come off nice and easily now.
If the skin doesn’t come off easily, it will shred a lot more (tomato on the right) as opposed to pulling off in large chunks (tomato on the left).
Cut the tomato in half around the equator then scoop or squeeze out the seeds.
To dice the tomato, cut in half again, lay flat, and cut into strips. Rotate the strips and cut again.