Miradouro do Recolhimento ("Recolhimento viewpoint") is a well-hidden secret in the neighborhood of Alfama. Situated just three minutes away from Castelo de São Jorge ("Saint George's Castle") and inaugurated in 2017, it is very underrated if compared to the other famous viewpoints in this neighborhood.
From here you can see the rest of the Alfama neighborhood, Igreja e Mosteiro de São Vicente de Fora ("Church and Monastery of St. Vincent Outside the Walls"), Panteão Nacional ("the National Pantheon") as well as the Tagus river from afar.
You'll notice that there are no clear signs to get to this public garden / viewpoint, so do rely on the GPS in your smart phone.