That flour needs to be well mixed with both a whisk and a rubber spatula into the butter. The flour needs to be cooked in that butter. Not just mixed in. You want to see it darken just a bit. Add just a little of the milk at a time. If it curdles it will get grainy. Keep cooking and mixing with both tools until you can use that rubber spatula to scrape the bottom and it takes a little while for the sauce to fill the void. Cook it longer still. You want it to be thick. Then add that cheese. But, again, not all at once. Three or four editions without adding more until it’s fully mixed in and melted.
That flour needs to be well mixed with both a whisk and a rubber spatula into the butter. The flour needs to be cooked in that butter. Not just mixed in. You want to see it darken just a bit. Add just a little of the milk at a time. If it curdles it will get grainy. Keep cooking and mixing with both tools until you can use that rubber spatula to scrape the bottom and it takes a little while for the sauce to fill the void. Cook it longer still. You want it to be thick. Then add that cheese. But, again, not all at once. Three or four editions without adding more until it’s fully mixed in and melted.