Well your question is difficult because of 3 reasons... this site is for Dubai specifically, what kind of lunch are you looking for and what area in Sharjah - but I will say for amazing seafood go to Seashells, best grill go to Mushmaool Arabic Grill Restaurant or for a casual lunch with international food options go to Shakespeare and Co.
@YO! Sushi they have a location over there good sushi not too expensive either. If you are willing to go a little farther @Minato is Dubai’s oldest Japanese restaurant and probably the most authentic, enjoy