Νοστιμιά is a relish or spice mix. In the case of the store brand, it's a powdered vegetable bouillon, often used in meat dishes. You may also see Νοστιμιά Μεσογειακή Σε Κόκκους (nostimia Mesogeiaki se kokkous) in the shops: a Mediterranean-style powdered bouillon in granules.
Some people make their own mixes, using ingredients such as mustard powder, turmeric, garlic powder, sweet paprika, powdered ginger and powdered oregano.
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