Preheat the oven to 425 F, and position a rack in the center of the oven. Line a standard muffin pan with 12 paper liners. Lightly spray the papers and the top of the pan with non-stick baking spray.
In a bowl, whisk together the flour, salt, baking soda, cinnamon and ginger.
Separately, whisk together the brown sugar, eggs, sour cream, melted butter and vanilla.
Add the wet ingredients and the diced peaches and rhubarb to the dry ingredients, and use a spatula to stir everything together, just until moistened.
Divide the batter between the muffin cups, filling them full.
In a bowl, stir together the flour, brown sugar and salt. Add the melted butter, and stir until moistened and crumbly.
Spoon the crumble on top of the batter. It may seem like too much crumble, but it's not. Just heap it on, and gently press it into the batter.
When you put the muffins in the oven, immediately turn the temperature down from 425 F to 375 F. Bake the muffins for about 22-25 minutes, until a toothpick or cake tester comes out clean.
Cool the muffins in the pan for 2 minutes, then gently lift them out and let them cool on a cooling rack for 5-10 minutes.
Enjoy the muffins warm, with softened butter.
