Line 12 muffin cups with large paper liners or lightly grease. Set aside.
In bowl, combine flour, brown sugar, granulated sugar, cinnamon, salt and pecans. Drizzle with melted butter and toss with fork until evenly moistened (mixture should be chunky).
In bowl, whisk together flour, pecans, baking powder, baking soda, lemon zest, cinnamon and salt. Set aside.
In separate bowl, combine egg and sugar and whisk until pale yellow in colour, about 30 seconds. Whisk in vanilla until thick, about 30 seconds. Slowly whisk in melted butter; add buttermilk and whisk until combined.
Fold flour mixture into wet ingredients, just until combined (over-mixing will make muffins dense and tough). Fold peaches gently into batter.
Divide batter into prepared muffin cups (will come almost up to top) and top evenly with streusel. Bake in centre of oven until light golden and toothpick inserted in centre comes out clean, about 30 minutes. Cool in tin for 10 minutes before transferring to rack to cool completely and serve or store in an airtight container at room temperature for up to 3 days.
Tip: Try substituting pecans with another nut, e.g., walnuts. Muffins may be frozen for up to 1 month; thaw at room temperature.
Per serving (1 muffin): about 290 calories, 4g protein, 14g total fat (5g saturated fat), 41g carbohydrates, 2g fiber, 40mg cholesterol, 220mg sodium. %RDI: iron 8%, calcium 6%, vitamin A 6%, vitamin C 2%