Farmer's market in Lisbon

There is a nice Mercado in Campo de Ourique. It is smaller than the Rebeira Mercado (Time Out), but still has several places to eat similar to Rebeira. Alvalede also has a Mercado according to my quick google search. I visited a Mercado during a cooking class two plus years ago and this may have been it. If so, they were adding a number of eating venues like Rebeira and Campo de Ourique.
If you make the trip to Cascais during your visit, come on Wednesday or Saturday. We have a large Mercado on those days.

We spent six weeks off and on in Estoril which is side by side with Cascais. We went to the market at least four times as we were staying with local residents. The fruit, vegetables, bakery items, fish and the most beautiful flowers were very good and inexpensive. Get there early! There are small cafes surrounding the market as well. There was also a wine festival while we were there where we tasted some very tasty wines and olive oils. The residents/vendors were very friendly and all spoke English for the most part. The Oceanside boardwalks were wonderful from Estoril to Cascais and we walked them often, with small bars and cafes to stop and enjoy a refreshment. We really enjoyed Lisbon and Cascais and made at least 4 train trips back and forth between the two. I highly recommend a day trip to Cascais, it is a very relaxing seaside town with very attractive buildings and shopping.