Heat the oven to 350°F with a rack in the center. Butter a 9-inch round metal cake pan, line the bottom with parchment, and lightly butter the parchment. Spread 42 g softened butter evenly across the base, then sprinkle with 100 g brown sugar. Toss the sliced pears with lemon juice, blot them again, and arrange them in a tight overlapping spiral. Keep the fruit in one even layer so the cake bakes level and releases without loose slices.
Line a rimmed baking sheet with parchment. Combine the sliced almonds, 30 g flour, 30 g brown sugar, and 0.5 g salt in a medium bowl. Pour in 28 g melted butter and stir until the flour is moistened and small clusters form. Spread the mixture in a loose layer and bake at 350°F for 12 to 15 minutes, stirring once, until golden and fragrant. Cool completely on the baking sheet; it will become crisper as the butter firms.
Whisk 150 g all-purpose flour, the almond flour, baking powder, and 3 g salt in a medium bowl for about 30 seconds. Break up any compacted almond flour with your fingertips or the whisk. The mixture should look uniform, with no visible pockets of leavening. Thorough distribution matters because almond flour contains no gluten and the wheat flour must carry the cake’s structure evenly around the moist pear layer.
Place 113 g softened butter and the granulated sugar in the bowl of a stand mixer fitted with the paddle. Beat on medium speed for 3 to 4 minutes, stopping once to scrape the bowl and paddle. The mixture should become pale, fluffy, and slightly increased in volume rather than greasy. Add the eggs one at a time on medium-low speed, mixing until each disappears, then blend in the vanilla and almond extracts.
Mix the sour cream into the butter mixture on low speed until mostly smooth. Add the dry ingredients in two additions, folding the final addition by hand with a silicone spatula. Stop when no dry flour remains; the batter should be thick, soft, and spoonable. Dollop it gently over the pears rather than pouring it into one spot. Use a small offset spatula to spread the batter to the edges without shifting the fruit, then smooth the top level.
Set the pan on a rimmed baking sheet to catch any caramel drips. Bake at 350°F for 43 to 48 minutes, rotating the pan after 30 minutes, until the cake is deeply golden, springs back when lightly pressed near the center, and begins pulling slightly from the sides. A skewer inserted into the cake portion should show moist crumbs but no wet batter. If the top browns early, cover it loosely with foil without sealing the pan.
Transfer the pan to a wire rack and cool for exactly 12 minutes. Run a thin offset spatula around the edge without cutting toward the pears. Place a large flat serving plate over the pan, protect your hands with dry oven mitts, and invert both pieces in one confident motion. Leave the pan in place for 30 seconds, then lift it straight up. Replace any attached pear slices while warm, cool for at least 30 minutes, and scatter the toasted almond crumb over the pears just before slicing.