Toss peaches & sugar in large bowl; let stand 30 min., tossing often. Drain peaches in colander, reserving juice. Whisk 1/4 cup reserved juice, cornstarch, lmeon juice, salt, cinnamon & nutmeg in small bowl. Toss with peaches and put in an 8” square glass baking dish.
While peaches are macerating, combine flour, sugars (reserve 1 Tbl. of the granulated sugar) and salt in bowl of food processor. Drizzle vanilla over and toss well. Add butter and half of the nuts and process until it clumps together into large crumbly balls. Sprinkle rest of nuts over and pulse twice. Transfer to parchment lined baking sheet and spread in even layer. The largest pieces should be roughly 1/2” chunks. Bake until lightly browned and firm (18-22 min).
Using parchment (with topping chunks) as funnel, slide the topping over peaches, spreading evenly and packing down lightly. Sprinkle reserved tablespoon of sugar over. Increase oven temp to 375 and bake until well browned and fruit is bubbling around edges, 25-35 minutes. Cool on wire rack at least 15 min.