What south American fruit tastes like a cross lemon and tommato?

The type of tomato you are looking for is called Lemon Boy "Lycopersicon Lycopersicum". However, the lemon taste is very mild but it is the closest you will get to a true lemon flavor that I can point you to that has a name. This only comes out when the tomato is fully and completely ripe and bears its signature pure bright yellow rind. There are some hybrids in the jungles of Central America that are a cross between Lemon boy and Roma the first exported American Tomato to Europe that has more lemon flavor but no scientist has given that particular plant a name yet. The flavor is very tart with a mild lemon flavor. Really only good for salsa or cooking as its flavor can be overwhelming for a sandwich.