© Denzil Green
A Garibaldi Biscuit is a a flat, rectangular cookie. Some describe it as a “cookie bar.”
There is a thin top and bottom layer, each of not overly sweet dough that is crisp when cooked. The top is glazed. Both layers have regularly spaced holes in them. Those are “docking” holes, like those in saltine crackers, to prevent the dough puffing up.
In the middle, there is a squashed currant filling. Because of this filling, generations of people have grown up calling the biscuits “squashed-fly biscuits”, “fly cemeteries”, “fly sandwiches”, or “dead fly biscuits” (which is similar to “dead fly pie”, a term used for Chorley Cake.)
The cookies are sold in strips of 5 cookies still attached to each other. Lines indicate where you break them.
There are several brands available. A version was made in the US by the “Sunshine Biscuit Company” with raisins. At various points, some companies have made chocolate covered ones — plain and milk chocolate, by makers such as Chiltonian biscuits. The Chiltonian Biscuit Factory in Hither Green, London made Garibaldis for Sainsburys and Peek Freaks in the 1970s.
Garibaldi Biscuits started off as a factory-baked cookie right from the start. A home recipe for them was created and presented by Tana Ramsay in her 2010 cookbook, “Tana’s Kitchen Secrets.” She uses a fork to reproduce the holes.
Garibaldi Biscuits were first sold in 1861.
They were created by the biscuit maker John Carr of Scotland — not though for the family biscuit business Carrs of Scotland. He had left them and gone to work for Peek Frean in Bermondsey, London.
The cookie is named after the man who unified Italy in the second half of the 1800s, Giuseppe Garibaldi, who had visited Britain for a month in 1854. 
Some think Garibaldi Biscuits may have been vaguely inspired by Chorley Cakes.
35 to 40 calories in each Garibaldi Biscuit
Wilson, Bee. The Kitchen Thinker: Garibaldi biscuits. Long live the Garibaldi! London: Daily Telegraph. 24 August 2010.
 “On 24 March 1854, the famous Italian revolutionary sailed into Tyneside, where, cutting quite a dash in his red silk shirt, poncho and sombrero, he was greeted with huge enthusiasm….. Garibaldi was so popular that some hotels even made a profit from selling his bathwater and hundreds of Italian café and tavern owners renamed their establishments after him. But the most lasting testatment to the boy-band level of hysteria that gripped Britain in 1854 was provided by cake makers Peek Frean, who later cooked a biscuit in his honour. Claiming it was based on the raisin bread he provided for his marching troops, they still produce the famous Garibaldi biscuits, or squashed fly biscuits, today.” — Jack, Albert. What Caesar Did For My Salad: The Secret Meanings of our Favourite Dishes. London: Penguin Books. 2010. Section: Garibaldi: A Very Revolutionary Biscuit. [Ed: Some sources maintain that if there actually had been any raisin bread, the reality was more likely dry bread smeared with a mixture of berries and horse blood consumed by his starving troops.]