I am in a different climate, SandraInSpain, and I hope you get some ideas from our Texas/Arizona gardeners. My climate is very much like the mountaneous north of yours (like in Burgos :o).

My grandmother was from Texas and raised a family in Oklahoma and New Mexico. She grew and liked Porter tomatoes. There are other, much more modern, varieties that I have not grown but Porter takes our arid, sunny summers here very well.

Thessaloniki is another heirloom tomato that I grow. As you can tell by the name, it was originally from Greece. It does just fine.

Growing advice? Keep the soil moist. Maybe afternoon shade would help.

Steve

Perhaps you can do two planting : One very early spring, as early as possible(say late March ). This way you will needs real nice size seedling (nursery grown or your own indoor grown). You should start picking ripe tomatoes around mid June(depends on variety). Then start another planting, like in late June-early july to bear fruits late summer and early fall.
That is how some gardeners do in Florida(USA). When the summer highs hit 38C(100F) most tomato plants either slow down quite a bit or even shut down production.

And I think DETERMINAT varieties are better for you.