Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with paper liners. Make the topping: Mix the oats, brown sugar, flour and salt in a small bowl. Add the butter and work it in with your fingers until the mixture comes together in clumps; set aside.
Make the muffins: Toss the rhubarb and strawberries in a small bowl with 1 tablespoon granulated sugar; set aside. Whisk the flour, remaining 3/4 cup granulated sugar, the baking powder, baking soda and salt in a large bowl; make a well in the center. Add the eggs to the well and gently whisk. Stir in the milk, melted butter and lemon juice until just combined with small bits of flour remaining. Stir in the rhubarb-strawberry mixture.
Divide the batter among the muffin cups, filling them most of the way. Top with the crumb mixture. Bake until the muffins are lightly browned and a toothpick inserted into the center comes out clean, 25 to 30 minutes. Let the muffins cool 5 minutes in the pan, then remove to a rack to cool completely. Dust with confectioners' sugar.