Literal Translation
To put butter in the spinach
Actual Meaning
To improve one’s life with the addition of money
Etymology
Butter is often associated with wealth (for example, ‘Faire son beurre’ [to make one’s butter], or ‘avoir le cul dans le beurre’ [to have one’s arse in butter] among our Walloon friends). This is probably because it is a rich food (in calories and fat) which, in certain troubled times, was reserved for the wealthy or black market champions.
The metaphor in this expression is perfectly understandable: spinach without butter is healthy but much less palatable than spinach with butter or cream.
So, to improve the taste of your spinach (your living conditions), it is better to add a generous helping of butter (money).