Source: Gourmet – Diary of a Foodie
adapted by http://theresalwaysthyme.blogspot.com/
6 to 8 ripe tomatoes
one 17- by 11-inch nonstick bakeware liner such as a Silpat and a sheet pan
an electric coffee/spice grinder
Preheat oven to 175ºF with rack in middle. Put liner in a 4-sided sheet pan.
Cut tomatoes crosswise into 1/8-inch-thick slices and arrange in 1 layer in pan.
Dry tomato slices in oven, turning over once, until completely dehydrated and crisp, 5 to 5 1/2 hours. Cool completely.
Crumble tomato into grinder and grind to a fine powder. Transfer to a fine-mesh sieve set over a bowl. Shake powder through sieve, then discard any pieces remaining in sieve.
Cooks’ note: Powder keeps in an airtight container at room temperature indefinitely.