To make galette dough, pulse all-purpose flour and salt in a food processor to combine. Add butter and pulse until mixture resembles coarse meal with a few pea-size pieces of butter remaining. Transfer mixture to a large bowl; drizzle with lemon juice and add sour cream. Mix with a fork, add ice water until dough comes together. You might not need all 5 tablespoons. Lightly knead until no dry spots remain and dough is smooth. Pat into a flat 1/2 inch thick disk and wrap in plastic. Chill at least 1 hour.
To make filling, in a sauté pan, heat oil over medium heat. Add onions and sauté for 7-8 minutes until onions have turned golden. Add vinegar and season with 1/2 tsp of salt and 1/2 tsp of pepper. stir and sauté for another 1-2 minutes until caramelized. Set aside to cool. In a medium bowl, mix together ricotta, 1 tbsp thyme, 1 tbsp honey and remaining salt and pepper.
Place apples and pear slices in a bowl, toss with remaining holes and thyme.
Preheat oven to 375F and line a large baking sheet with parchment paper.
Remove dough from refrigerator and let rest to soften for a few minutes. Dust clean surface with flour and roll out dough to 14 inches round with even thickness. Carefully transfer to lined backing sheet. Spread ricotta cheese evenly on dough, leaving about 2 inches around the edges. Spread onions evenly on ricotta and lay pears and apples evenly and slightly overlapping onto. Drizzle any juices from the bowl onto.
Fold in the sides just enough to meet up with the apples and pears to make a frame around the them.
Brush the frame of the dough with the egg wash and sprinkle the sesame seeds ( if using) on top of egg wash around the pastry frame.
Place in the oven on the middle rack and bake for 45-50 minutes until crust is dark golden and fruit has softened. Let cool for 5 minutes prior to serving. You can also drizzle additional honey on top.
