Almost every restaurant of quality caliber can serve a single / solo diner. It might be at the bar or it might be a smaller table. The key might involve getting that reservation which can be based on making it 2-3 months before the actual meal. If you want a foodie type place then Resy / Tock are better than opentable for usa. For outside USA one must check the platforms which are based on location. So my knowledge is strictly usa.