Preheat the oven to 425 ºF. Line two muffin pans with 8 to 10 muffin paper liners being sure to space them out between the two pans to allow better air flow.
Make the streusel topping
In a small bowl, whisk together the flour, ground almond, sugar, salt, and cinnamon (if using).
Add the melted butter and stir it in with a fork until the mixture forms a crumble.
Cover and store in the refrigerator until you are ready to use it.
Make the muffin batter
In a large bowl, whisk together the flour, sugar, ground almond, baking powder, baking soda, and salt.
Add the softened butter and work it in with your hands, rubbing it in until the mixture resembles coarse sand.
In a separate bowl, whisk together the sour cream, eggs, and vanilla.
Add the wet ingredients to the flour mixture and stir it in (takes about 20 stirs to get the batter mixed).
When the batter is ready, in a medium bowl, toss the rhubarb (and the strawberries, if using) with the 8 grams (1 tbsp) of flour. Stir so that the the fruits are evenly coated with flour.
Fold them into the batter gently with a wooden spoon or spatula to evenly distribute them.
Divide the batter between 8 to 10 paper-lined wells of two muffin pans. If you want big muffins, you'll need 8, if you want regular-sized muffins, you'll need 10. Sprinkle with streusel.
Bake at 425°F for 20 minutes, then use a cake tester inserted into the middle of a muffin to check if the muffins are baked. If you are baking 8 muffins, they probably aren't baked through, so rotate the pans and drop the temperature setting to 350°F for the last 5 to 10 minutes or as much as necessary.
For a “regular” muffin top: bake at 350°F for 25 to 30 minutes. For a “domed” muffin top: bake at 425°F for 20 minutes, then rotate the pans and drop the temperature setting to 350°F for the last 5 to 10 minutes. For a “puffy” muffin top: refrigerate the batter overnight, then scoop the batter into 8 to 10 muffin paper-lined wells of a muffin pan, sprinkle with streusel topping, and bake them at 350°F for about 30 minutes.