This cartoon by Paul Thomas from the Daily Express relates to the continuing horsemeat scandal, which has shocked consumers in Britain and across the EU over the past two weeks. The scandal, which has triggered product recalls and damaged confidence across Europe's food industry, erupted last month when tests carried out in Ireland revealed that some beef products also contained horsemeat.
In the cartoon, a greengrocer assures a customer that there is no horse in his beef tomatoes. Beef tomatoes (see pictures here) are a type of very large tomato. The joke is that the woman is so worried about finding horsemeat in beef products that she asks the shopkeeper for reassurance. Of course, beef tomatoes do not contain beef; the name is probably because of the texture and taste. It's a fleshy tomato with a very ripe and firm texture.
A greengrocer is a shopkeeper who sells fruit and vegetables. Strictly speaking, the shop sign should have an apostrophe: greengrocer's (as with other shops such as chemist's, baker's, fishmonger's, etc.). Ironically, there is something called the greengrocer's apostrophe, which is an apostrophe erroneously inserted before the final "s" in the plural form of a word, as in "Apple's 55p per Ib" and "We sell the original shepherds pie's" (notice in a shop window, Canterbury, England).