Slime Taikou to Botsuraku Reijou no Angai Shiawase na Konyaku

After her sister's engagement to the crown prince is broken off suddenly, tragedy strikes, and Lady Francette and her family lose all of their wealth and status. Left with nothing but her name and determination, Francette moves to the slums and ekes out a living selling homemade pastries. That's when she meets an unlikely new friend named Wibble, a talking slime who introduces her to the one and only slime duke himself: Gabriel! To her surprise, the unusually stoic Gabriel soon asks for Francette's hand in marriage—and she accepts. Now she must adapt to Triste and its eccentric people, and she soon finds herself breathing life into the strange but endearing backwater. What was once a marriage of convenience becomes something much more in time...and—of course—expect lots of slime!

(Source: J-Novel Club)