Solar produces power even on cloudy days. So the simple answer is you overbuild solar to produce enough power even on cloudy days.

That requires about a 10x overbuild. In reality you do about a 3x overbuild, exporting to the cloudy places in the rest of Europe when you are cloudy and importing from the sunny places when you are cloudy. It's sometimes cloudy in most of Europe but it's never cloudy in all of Europe.

Then you do a similar thing with wind. Wind and solar are anti-correlated.

You can also make it easier by not shutting down existing nuclear. New nuclear is horribly expensive, but keeping existing plants running is cost effective.