Start by gathering the flour, instant yeast, sugar, and salt in a bowl. Stir them together so everything is evenly blended.
Warm water and melted butter (or oil) go in next. Work everything together until a rough dough comes together. Move it to the counter and knead by hand for about 5–7 minutes until the surface becomes smooth and elastic.
Work the cheddar and jalapeños into the dough, pressing and folding until they’re spread throughout.
Shape the dough into a round and return it to the bowl. Cover it with a towel and leave it to rise for about an hour, until it looks noticeably fuller.
Deflate the dough gently, shape it into a loaf, and set it in a greased pan—or place it on a baking sheet if you prefer a free-form loaf. Give it a short rest of about 10–15 minutes while the oven heats to 190°C.
Slide the dough into the oven and bake for 30–35 minutes. The top should turn golden, and the bottom should give a hollow sound when tapped. Set the loaf on a rack to cool before cutting.