Many restaurants in Spain are closed on Sunday nights (and Mondays) so I would plan on having lunch (comida) rather than dinner if you have particular places in mind. A bigger problem will be where to eat on the following day, Monday.
One of the best restaurants in Ronda is Bardal; they are now taking reservations for Easter lunch but it is a tasting menu. I imagine you might run into the situation in many places--open for Easter Sunday lunch but with a special holiday menu. (Bardal always offers a a choice of two tasting menus)
The same chef haas a less expensive restaurant, Tragata, that is also open for Easter lunch; here is the menu; prices are reasonable and they offer a range of steaks which are probably very good. If I were in your place, I'd probably go there and be sure to book well inn advance of arriving in Spain. They are taking bookings now.
https://www.tragata.com/_files/ugd/30a80e_3f00cc7646ed4101b217e5f0b9552361.pdf
But where are you sleeping? Are you on a tour that does not allow a long Sunday lunch? if so, the issue will be more difficult. Does your tour itinerary not tell you the hotel and town where you will spend Sunday night?
It's easy enough to check others.....just gather a list of restaurants that interest you and check their websites to see if you can book on April 20 for dinner. Difficult, I think, not only cause it is a Sunday but it's the holiday Sunday.