Restaurants in Madrid usually offer inexpensive (10-15€) lunch menus (“Menú del día”) from 1:30 pm to 3:30 pm (they are not normally announced on their websites). You can easily reach La Latina neighborhood (at the west side of Toledo street) on foot, where you will find a lot of restaurants offering this sort of menus as well as other lunch options. If you want to have a drink or dinner after the conference, La Latina might be also a good choice (many bars that are closed at lunch are open from 7:00 pm onwards).
The following map shows a selection of restaurants and coffee shops nearby the conference venue. If you zoom in, Google will give you much more exhaustive information: