Halve, pit and coarsely chop plums. In a saucepan over low heat, combine the plums and sugar. Cook, stirring, until the plums have released their juices and the sugar is dissolved, 5 to 8 minutes. Bring to a simmer and cook, uncovered, until the mixture thickens and the plums have a jamlike consistency, 25 to 30 minutes. Stir in the lemon juice. Let cool to room temperature, then cover and refrigerate for up to 2 weeks.
Preheat a nonstick panini maker according to the manufacturer’s directions. Spread 1 to 2 Tbs. of the almond butter on each of 2 slices of bread. Spoon about 2 Tbs. of the ricotta over the almond butter. Spread about 1 Tbs. of the plum preserves on each of the remaining 2 slices of bread. Assemble the sandwiches and toast in the panini maker until crisp and golden, 3 to 5 minutes. (If you have a panini maker that allows you to exert pressure to flatten the sandwich, don’t compress too much, or you will lose some delicious filling.) Serve immediately. Makes 2 sandwiches.