1. Preheat the oven to 220°C. In a large bowl, mix the flour and salt. Rub in the butter with your fingers until crumbly.
2. Add the 2 cheeses and sage and mix thoroughly. Stir in the milk gradually until you have a dough. If the mixture is sticky, add a little more flour. If the mixture seems too dry and isn't holding together, moisten with a little more milk until cohesive but not sticky.
3. Turn out on a clean work surface and knead until fairly smooth for 2-3 minutes. Squash the dough into a flattened round approximately 18cm in diameter and 4cm thick. Use a long, sharp knife to cut almost all the way through the dough 4 times, making 8 wedges.
4. Brush the top surface with milk and sprinkle with poppy seeds. Bake for about 25 minutes, until golden and firm. Cool on a wire rack and eat warm, cold, or toasted, with butter.