Add the onion, carrot and celery to a food processor and blitz until finely chopped.
Chop the tempeh block very finely until you reach a mince kind of consistency.
Heat the oil in a large pan over a medium heat, add the blitzed up veggies and cook for about 6-8 minutes until starting to soften. Add the garlic and cook for another minute, stirring to prevent from burning.
Add the tempeh to the pan and cook for 4-5 minutes.
Add the chopped tomatoes, tomato paste, salt, herbs and yeast extract to the pan. Stir well and bring it to a boil, then cover and simmer for 20-25 minutes on a very gentle heat. If the sauce becomes too thick add a splash of water.
Serve over the pasta of your choice, with cracked black pepper and a sprinkle of chilli flakes if desired.