Mix together ricotta cheese, lemon zest, vanilla, and honey. Set aside.
Place sliced plums in a large bowl and add in sugar, cornstarch, and cinnamon. Toss gently with your hands until well combined.
Grease an 11-inch fluted tart pan with removable bottom. Place one fillo sheet in the pan. Edges of fillo should hang over edges of the pan. Lightly brush the fillo sheet with butter. Make sure to brush the hanging sides as well. Continue to layer three more sheets of fillo brushing each layer (and sides) with butter and rotating sheets to covert he pan.
Pour ricotta mixture into the tart. Arrange plum slices in rows on top. Sprinkle with sliced almonds, if using.
Gently fold the overhanging fillo up over the top of the tart (see recipe notes) and brush with remaining butter.
Bake until fillo is golden brown and plums are soft, about 20-30 minutes. Let cool slightly before carefully removing the tart from mold, slicing, and serving.
You can use any kind of plums. I used Santa Rosa.
If your plums are firm, you can use a mandoline to slice them about ⅛ inch thick.
Arrange your plums any way you want. Or don't, for a more rustic look.
Have all your ingredients ready before handling the fillo pastry. Work fast as it dries quickly and becomes brittle. Keep the rest of fillo tightly wrapped and refrigerated until next use (see recipe suggestions).