In a 9-inch ovenproof skillet (not nonstick), simmer honey until it begins to caramelize and darken in colour, do not let honey burn; if it starts to smell burned, turn off heat.
Arrange pears, close together and cut-side down, in a circular pattern in skillet, stem ends pointing toward center. Simmer over medium heat, turning them from one side to the other until they turn golden, about 10 minutes.
Flip pears over their curved side and scatter with thyme sprigs if using. Transfer skillet to oven and roast, uncovered, until very tender, about 25 minutes.
Meanwhile, in a large bowl, whisk together sugar and lemon zest. Whisk in eggs and vanilla, fold in flour and salt, stir in 1/2 cup of butter.
When pears are soft, remove skillet from oven, discard thyme sprigs and brush edges of pears with the remaining 1 tbsp of butter. Pour batter over pears, spread almonds over batter and bake for about 25-30 minutes until done. Let cake cool for 30 minutes in the skillet before running an offset spatula or a thin sparing knife along edge of pan to loosen cake. Carefully invert cake into a platter. Serve warm or cooled.