Line a 6x4 inch loaf tin with non-stick baking paper. Preheat the oven to 175 C. (An 8 inch round cake tin will work too and you could double the recipe for a standard 9x5 loaf).
Using a fine grater (or zester, if you have it), zest the orange into a mixing bowl, making sure you don't grate down to the bitter white pith. Slice the orange in half and squeeze out the juice into a smaller bowl (you'll have about 1/4 cup juice). Set aside.
Add the butter and sugar to the mixing bowl with the orange zest. Beat with a hand mixer on low speed until light and creamy. Add the egg and vanilla and beat till combined.
Add the ground almonds and semolina, beating briefly to combine. Then sift in the flour, salt and baking soda. Beat on low speed or stir gently by hand until the flour is incorporated.
Pour in the orange juice and beat till you have a smooth, fairly thick batter. Transfer to the prepared loaf tin and smoothen the top.
Bake for 45 to 50 minutes until browned on top (the center will have a small crack) and a toothpick poked in the center comes out clean.
In the last ten minutes of baking time, combine the syrup ingredients in a small saucepan and bring to a boil for 8 to 10 minutes on low heat, until it thickens. Set aside for a few minutes to let it thicken further.
Poke holes into the cake (while it's still hot) and pour the slightly cooled syrup all over it, spreading it around as you go. Let the cake cool completely in the tin itself, preferably overnight at room temperature. While it may not soak all the way through, the syrup makes the cake super flavourful and moist so don't skip it!
When fully cooled and the syrup looks set on the top, slice and dig in! The cake is fragile because of the almonds and semolina which don't have as much structure as using only all-purpose flour, so the center pieces might be a little harder to cut neatly because that's typically where loaf cakes develop a crack. If you're concerned, you can also chill the cake for about 30 minutes before slicing it, it tastes great cold!
Store at room temperature for 3 to 4 days in an airtight tin, and in the fridge for a week. It can also be frozen for longer. Happy baking!