Use a whisk to combine the Nature Valley Toasted Oats Muesli, brown sugar, wheat flour, and salt in a medium large mixing bowl that is heat safe. Drop the pat of butter on top, pour the boiling water in, and whisk until the sugar is dissolved and the butter is melted. Cover the bowl and let it come to room temperature. Stir in the warm water and the active dry yeast until evenly combined. Add the all-purpose flour to the bowl and stir with a sturdy spoon until you have a shaggy dough. Turn out onto a generously floured work surface. Knead, adding flour 1 tablespoon at a time, until you have a smooth, cohesive, still slightly tacky dough. Put into a mixing bowl (it can be the bowl you used to mix up the dough, scraped clean), cover with a damp tea towel, and leave undisturbed to rise until double in size, about an hour, depending on room temperature and humidity.