Position a rack in the center of the oven and heat the oven to 325 degrees F.
In a stand mixer fitted with the paddle attachment, mix the flour, oats, butter, almonds, sugars, thyme, and salt on medium speed until the ingredients are evenly dispersed and some of the pieces of streusel are the size of hazelnuts. Transfer the streusel to a large rimmed baking sheet and bake, stirring every 5 minutes to 6 minutes, until light golden brown, about 20 minutes. Let the streusel cool completely before using.
Put 3 ounces of the white chocolate in a medium bowl. Bring 1/2 cup of the cream and the lemon zest to a boil in a small saucepan over medium-high heat; pour over the chocolate. Let sit for a few minutes, then whisk until smooth. Stir in the lemon juice and salt, cover the bowl tightly, and refrigerate until cold, at least 1 hour and up to 24 hours.
Add the remaining 1/2 cup cream to the chilled mixture and beat with an electric hand mixer on medium speed until stiff peaks form, about 2 minutes. Refrigerate in an airtight container until ready to use.
Put the blueberries, sugar, cornstarch, cinnamon, and salt in a 12-inch heavy-duty skillet over medium heat. Using a potato masher or a silicone spatula, break up some of the blueberries to release their juice and cook, stirring often, until the mixture begins to boil, about 5 minutes. Once the mixture comes to a boil, stir constantly, reducing the heat to maintain a steady simmer, until thickened, about 2 minutes. Remove the pan from the heat and stir in the butter and lemon juice until melted. Let the filling cool until warm.
Put 1 tablespoon of the streusel in the bottom of each of six 10 ounce parfait glasses. Divide the warm filling among them and scatter each with about 2 tablespoons streusel. Divide the remaining filling among the glasses and top with the remaining streusel. Dollop each with the white chocolate cream, sprinkle with the remaining chopped white chocolate and serve.